feat(samples): add "Overshoot and collapse" gallery sample

The twelfth sample fills the one classic dynamic the gallery was missing —
overshoot and collapse, the dynamic Thinking in Systems is built around. A
Reinforcing engine (Capital reinvesting its extraction revenue) runs on a
non-renewable Resource, the first Stock in the gallery with no inflow: it
overshoots the limit instead of settling at it, the dark twin of "Limits to
growth".

Buildable with the existing rule vocabulary (proportional only); the
non-negative-stock floor tames the bilinear extraction term as the Resource
runs out. Tuned against the simulator: Capital 5 → ~250 (t≈39) → ~5 by t=150,
Resource 1000 → ~6, no divergence. The loop detector classifies the three
loops as expected (R: investment→Capital→extraction; B: depreciation→Capital;
B: extraction→Resource).

public/favicon.svg

@@ -4,7 +4,7 @@
   height="24"
   viewBox="0 0 24 24"
   fill="none"
-  stroke="#16A34A"
+  stroke="#ff9ff3"
   stroke-width="2"
   stroke-linecap="round"
   stroke-linejoin="round"

src/components/Editor.vue

@@ -349,7 +349,28 @@ onBeforeUnmount(() => {
 <template>
   <div class="flex h-screen flex-col bg-base-200">
     <header class="flex items-center gap-3 border-b border-base-300 bg-base-100 px-4 py-2">
-      <img src="/favicon.svg" alt="" class="size-6" />
+      <!-- Inlined favicon so the logo strokes with the theme primary (currentColor
+           via text-primary); an external <img> can't read the theme var. -->
+      <svg
+        class="size-6 text-primary"
+        viewBox="0 0 24 24"
+        fill="none"
+        stroke="currentColor"
+        stroke-width="2"
+        stroke-linecap="round"
+        stroke-linejoin="round"
+        aria-hidden="true"
+      >
+        <path d="M14 20a2 2 0 1 0 -4 0a2 2 0 0 0 4 0" />
+        <path d="M14 4a2 2 0 1 0 -4 0a2 2 0 0 0 4 0" />
+        <path d="M6 12a2 2 0 1 0 -4 0a2 2 0 0 0 4 0" />
+        <path d="M22 12a2 2 0 1 0 -4 0a2 2 0 0 0 4 0" />
+        <path d="M14 12a2 2 0 1 0 -4 0a2 2 0 0 0 4 0" />
+        <path d="M6 12h4" />
+        <path d="M14 12h4" />
+        <path d="M12 6v4" />
+        <path d="M12 14v4" />
+      </svg>
       <h1 class="text-lg font-semibold">meadows</h1>
       <span class="text-sm text-base-content/50">
         {{ store.nodeCount }} {{ store.nodeCount === 1 ? "element" : "elements" }}

src/components/Inspector.vue

@@ -62,6 +62,12 @@ function onInitial(event: Event): void {
   if (Number.isFinite(n)) store.setInitialValue(el.id, n)
 }
 
+function onUnit(event: Event): void {
+  const el = element.value
+  if (el?.kind !== "stock") return
+  store.setUnit(el.id, (event.target as HTMLInputElement).value)
+}
+
 function onKind(event: Event): void {
   const el = element.value
   if (el?.kind !== "flow" && el?.kind !== "converter") return
@@ -97,17 +103,29 @@ const RULE_HINT: Record<Rule["kind"], string> = {
       <span class="text-xs text-base-content/50">{{ KIND_LABEL[element.kind] }}</span>
     </div>
 
-    <!-- Stock: the quantity it starts from. -->
-    <label v-if="element.kind === 'stock'" class="mt-2 block">
-      <span class="text-xs text-base-content/60">Initial value</span>
-      <input
-        type="number"
-        class="input input-sm input-bordered mt-1 w-full"
-        :value="element.initialValue ?? ''"
-        placeholder="—"
-        @change="onInitial"
-      />
-    </label>
+    <!-- Stock: the quantity it starts from, and its unit. -->
+    <template v-if="element.kind === 'stock'">
+      <label class="mt-2 block">
+        <span class="text-xs text-base-content/60">Initial value</span>
+        <input
+          type="number"
+          class="input input-sm input-bordered mt-1 w-full"
+          :value="element.initialValue ?? ''"
+          placeholder="—"
+          @change="onInitial"
+        />
+      </label>
+      <label class="mt-2 block">
+        <span class="text-xs text-base-content/60">Unit</span>
+        <input
+          type="text"
+          class="input input-sm input-bordered mt-1 w-full"
+          :value="element.unit ?? ''"
+          placeholder="e.g. °C, people, $"
+          @change="onUnit"
+        />
+      </label>
+    </template>
 
     <!-- Flow / Converter: pick a rule, then its number. -->
     <template v-else>

src/components/ResultsPanel.vue

@@ -134,7 +134,9 @@ const chart = computed(() => {
           aria-label="Playhead"
           @input="sim.seek(Number(($event.target as HTMLInputElement).value))"
         />
-        <span class="w-14 text-right font-mono text-xs tabular-nums text-base-content/60">
+        <span
+          class="w-24 shrink-0 whitespace-nowrap text-right font-mono text-xs tabular-nums text-base-content/60"
+        >
           t = {{ sim.currentTime ?? 0 }}
         </span>
       </div>

src/components/SimChart.vue

@@ -43,6 +43,27 @@ let observer: ResizeObserver | undefined
 /** Signature of the current plot's tracks; a change means the shape changed. */
 let builtSig = ""
 
+// Playhead as a DOM overlay (CSS px), not a canvas line: a moving line drawn by
+// redrawing the whole uPlot canvas every frame flickers, so the line slides over
+// the canvas instead and the canvas is only redrawn when the data changes.
+const markerX = ref<number | null>(null)
+const markerTop = ref(0)
+const markerHeight = ref(0)
+
+function syncMarker(): void {
+  if (!plot || props.marker === null) {
+    markerX.value = null
+    return
+  }
+  // bbox is in device pixels; the overlay is laid out in CSS pixels.
+  const ratio = window.devicePixelRatio || 1
+  // valToPos (CSS px) is relative to the plot area's left edge, but the overlay
+  // is positioned against the canvas, so add the left gutter (the y-axis width).
+  markerX.value = plot.bbox.left / ratio + plot.valToPos(props.marker, "x")
+  markerTop.value = plot.bbox.top / ratio
+  markerHeight.value = plot.bbox.height / ratio
+}
+
 /** Compact axis/legend numbers (7039.99 → "7040", 0.42 → "0.42", idle → "--"). */
 function fmt(value: number | null): string {
   if (value === null) return "--"
@@ -76,26 +97,6 @@ function options(width: number): uPlot.Options {
     height: props.height,
     cursor: { y: false },
     scales: { x: { time: false } },
-    // Playhead: a vertical line at the current simulation time, so the chart and
-    // the animating canvas read off the same instant. Drawn after the series, and
-    // redrawn whenever `marker` changes (see the watch below).
-    hooks: {
-      draw: [
-        (u: uPlot): void => {
-          if (props.marker === null) return
-          const x = Math.round(u.valToPos(props.marker, "x", true))
-          const { ctx } = u
-          ctx.save()
-          ctx.beginPath()
-          ctx.lineWidth = 1
-          ctx.strokeStyle = themeColor("--color-primary", "#2563eb")
-          ctx.moveTo(x, u.bbox.top)
-          ctx.lineTo(x, u.bbox.top + u.bbox.height)
-          ctx.stroke()
-          ctx.restore()
-        },
-      ],
-    },
     series: [
       {},
       ...props.series.map(
@@ -123,17 +124,25 @@ function build(): void {
 function render(): void {
   if (!plot || trackSig() !== builtSig) build()
   else plot.setData(props.data)
+  syncMarker() // the plot area may have shifted; keep the overlay aligned
 }
 
 onMounted(() => {
   build()
   observer = new ResizeObserver(() => {
-    if (plot && root.value) plot.setSize({ width: root.value.clientWidth, height: props.height })
+    if (plot && root.value) {
+      plot.setSize({ width: root.value.clientWidth, height: props.height })
+      syncMarker()
+    }
   })
   if (root.value) observer.observe(root.value)
+  syncMarker()
   // The canvas paints before the web font loads and won't repaint on its own, so
   // axis labels would stick to the fallback. Redraw once the font is ready.
-  document.fonts?.ready.then(() => plot?.redraw())
+  document.fonts?.ready.then(() => {
+    plot?.redraw()
+    syncMarker()
+  })
 })
 
 onBeforeUnmount(() => {
@@ -142,15 +151,18 @@ onBeforeUnmount(() => {
 })
 
 watch(() => props.data, render)
-// The playhead moves without the data changing, so nudge a redraw on its own.
-watch(
-  () => props.marker,
-  () => plot?.redraw(),
-)
+// The playhead just slides the overlay — no canvas redraw, so playback stays smooth.
+watch(() => props.marker, syncMarker)
 </script>
 
 <template>
-  <div ref="root" class="w-full" />
+  <div ref="root" class="relative w-full">
+    <div
+      v-if="markerX !== null"
+      class="pointer-events-none absolute w-px bg-primary"
+      :style="{ left: `${markerX}px`, top: `${markerTop}px`, height: `${markerHeight}px` }"
+    />
+  </div>
 </template>
 
 <style scoped>

src/components/nodes/StockNode.vue

@@ -37,17 +37,24 @@ const fill = computed(() => sim.fill(props.id))
       class="pointer-events-none absolute inset-[2px] z-0 flex flex-col justify-end"
     >
       <div
-        class="rounded-b bg-primary/40 transition-[height]"
+        class="rounded-b bg-primary/60 transition-[height]"
         :style="{ height: fill * 100 + '%' }"
       />
     </div>
     <Handle :id="HANDLE_IN" type="target" :position="Position.Left" />
     <div class="relative z-10">
       <NodeLabel :node-id="props.id" :name="stock.name" />
-      <div v-if="value !== null" class="mt-0.5 font-mono text-xs tabular-nums text-base-content/70">
-        {{ formatValue(value) }}
-      </div>
     </div>
     <Handle :id="HANDLE_OUT" type="source" :position="Position.Right" />
+    <!-- Live value sits *below* the box as an out-of-flow overlay, so the box keeps
+         its exact size and the handles — with the pipes and links on them — never
+         shift when a run is engaged. -->
+    <div
+      v-if="value !== null"
+      class="pointer-events-none absolute inset-x-0 top-full mt-1 whitespace-nowrap text-center font-mono text-xs tabular-nums text-base-content/70"
+    >
+      {{ formatValue(value)
+      }}<span v-if="stock.unit" class="ml-0.5 text-base-content/50">{{ stock.unit }}</span>
+    </div>
   </div>
 </template>

src/model/io.ts

@@ -89,6 +89,9 @@ function nodeError(value: unknown, index: number): string | null {
   if (kind === "stock" && value.initialValue !== undefined && !isFiniteNumber(value.initialValue)) {
     return `${at}.initialValue must be a finite number`
   }
+  if (kind === "stock" && value.unit !== undefined && typeof value.unit !== "string") {
+    return `${at}.unit must be a string`
+  }
   if (
     (kind === "flow" || kind === "converter") &&
     value.rule !== undefined &&

src/model/samples.ts

@@ -33,6 +33,14 @@
  *  11. Drift to low performance — a goal that erodes toward actual performance, so a
  *                                 Reinforcing loop ratchets both downward.
  *
+ * Last, the dynamic the book is named for, and the one the gallery has saved until a
+ * reader knows every piece it needs:
+ *
+ *  12. Overshoot and collapse   — a Reinforcing engine running on a *non-renewable*
+ *                                 Stock (the first with no inflow): it overshoots the
+ *                                 limit instead of settling at it, the dark twin of
+ *                                 "Limits to growth" — the ceiling erodes, so it crashes.
+ *
  * These are plain data built from the same tested constructors the store uses
  * (factory.ts), so every sample is a valid Model by construction. `build()`
  * mints fresh ids on each call, so loading a sample twice never collides.
@@ -77,6 +85,7 @@ function bathtub(): Model {
   const source = makeCloud({ x: -280, y: 0 })
   const water = makeStock({ x: 0, y: 0 }, "Water")
   water.initialValue = 20
+  water.unit = "L"
   const sink = makeCloud({ x: 280, y: 0 })
   const filling = makeFlow(
     midpoint(source.position, water.position),
@@ -108,6 +117,7 @@ function savings(): Model {
   const source = makeCloud({ x: -240, y: -80 })
   const balance = makeStock({ x: 120, y: 40 }, "Balance")
   balance.initialValue = 1000
+  balance.unit = "$"
   const interest = makeFlow(
     midpoint(source.position, balance.position),
     "interest",
@@ -133,6 +143,7 @@ function savings(): Model {
 function coffee(): Model {
   const coffee = makeStock({ x: -200, y: 0 }, "Coffee")
   coffee.initialValue = 90
+  coffee.unit = "°C"
   const sink = makeCloud({ x: 200, y: 0 })
   const cooling = makeFlow(midpoint(coffee.position, sink.position), "cooling", coffee.id, sink.id)
   // cooling = 0.1 × (Coffee − room): the `+` input is the level, the `−` the target.
@@ -169,6 +180,7 @@ function population(): Model {
   lifeExpectancy.rule = { kind: "constant", value: 70 }
   const people = makeStock({ x: 0, y: 0 }, "Population")
   people.initialValue = 100
+  people.unit = "people"
   const births = makeFlow({ x: -160, y: -160 }, "births", source.id, people.id)
   // births = fertility × Population (both `+` inputs): more people and higher
   // fertility, more births — the Reinforcing engine.
@@ -206,6 +218,7 @@ function limitsToGrowth(): Model {
   const source = makeCloud({ x: -280, y: 0 })
   const yeast = makeStock({ x: 40, y: 0 }, "Yeast")
   yeast.initialValue = 20
+  yeast.unit = "cells"
   const growth = makeFlow(midpoint(source.position, yeast.position), "growth", source.id, yeast.id)
   // growth = 30% of Yeast (its `+` input): the Reinforcing engine.
   growth.rule = { kind: "proportional", factor: 0.3 }
@@ -321,10 +334,13 @@ function predatorPrey(): Model {
 function epidemic(): Model {
   const susceptible = makeStock({ x: -280, y: 0 }, "Susceptible")
   susceptible.initialValue = 990
+  susceptible.unit = "people"
   const infected = makeStock({ x: 0, y: 0 }, "Infected")
   infected.initialValue = 10
+  infected.unit = "people"
   const recovered = makeStock({ x: 280, y: 0 }, "Recovered")
   recovered.initialValue = 0
+  recovered.unit = "people"
   const infection = makeFlow(
     midpoint(susceptible.position, infected.position),
     "infection",
@@ -563,6 +579,68 @@ function driftToLowPerformance(): Model {
   )
 }
 
+/**
+ * Overshoot and collapse — the dark twin of "Limits to growth". The same
+ * Reinforcing engine runs, but the limit here is a *non-renewable* Resource that
+ * only depletes: a Stock with no inflow, the first in the gallery. An economy
+ * (Capital) lives off it — extraction grows with both the Resource left and the
+ * Capital deployed (Resource, Capital → [+] → extraction), and the revenue is
+ * reinvested as new Capital (extraction → [+] → investment → Capital), so the loop
+ * Capital → extraction → investment → Capital carries no `−` → Reinforcing. Capital
+ * climbs and extraction accelerates, but every unit burned is gone for good, so the
+ * Resource crosses the break-even level, the engine starves, and depreciation
+ * (Capital → [+] → depreciation, a Balancing drain) takes Capital down: it peaks,
+ * then collapses. Contrast "Predator and prey", whose prey regrows and so settles
+ * into oscillation — a finite Resource cannot, so it overshoots and crashes instead.
+ */
+function overshootAndCollapse(): Model {
+  // Resource on the left drains only downward (no inflow). Capital on the right runs
+  // a full Source → investment → Capital → depreciation → Sink column. The two
+  // coupling links — Capital → extraction and extraction → investment — cross in the
+  // open centre, where the R badge lands.
+  const resource = makeStock({ x: -240, y: 0 }, "Resource")
+  resource.initialValue = 1000
+  const extractionSink = makeCloud({ x: -240, y: 360 })
+  const extraction = makeFlow({ x: -240, y: 160 }, "extraction", resource.id, extractionSink.id)
+  // extraction = factor × Resource × Capital (both `+`): more capital extracts
+  // faster, scarcer resource slower. The bilinear term the non-negative floor tames.
+  extraction.rule = { kind: "proportional", factor: 0.0004 }
+  const capital = makeStock({ x: 240, y: 0 }, "Capital")
+  capital.initialValue = 5
+  const investmentSource = makeCloud({ x: 240, y: -360 })
+  const investment = makeFlow({ x: 240, y: -160 }, "investment", investmentSource.id, capital.id)
+  // investment = factor × extraction (its one `+` input): the revenue reinvested —
+  // a Flow feeding a Flow, the edge that closes the Reinforcing loop through Capital.
+  investment.rule = { kind: "proportional", factor: 0.5 }
+  const depreciationSink = makeCloud({ x: 240, y: 360 })
+  const depreciation = makeFlow({ x: 240, y: 160 }, "depreciation", capital.id, depreciationSink.id)
+  // depreciation = factor × Capital (its `+` input): the Balancing drain that wins
+  // once the Resource can no longer feed investment.
+  depreciation.rule = { kind: "proportional", factor: 0.04 }
+  return model(
+    "Overshoot and collapse",
+    [
+      resource,
+      extractionSink,
+      extraction,
+      capital,
+      investmentSource,
+      investment,
+      depreciationSink,
+      depreciation,
+    ],
+    [
+      link(resource, extraction, "+"),
+      link(capital, extraction, "+"),
+      link(extraction, investment, "+"),
+      link(capital, depreciation, "+"),
+    ],
+    // Capital starts at 5, overshoots to ~250 by t≈39, and collapses back near its
+    // starting level by t=150 — the full boom-and-bust arc, no dead tail.
+    { start: 0, stop: 150, dt: 1 },
+  )
+}
+
 /** The gallery, ordered simplest first. */
 export const SAMPLES: Sample[] = [
   { title: "Bathtub", blurb: "A stock filled and drained — no feedback yet.", build: bathtub },
@@ -616,4 +694,9 @@ export const SAMPLES: Sample[] = [
     blurb: "Goals erode toward actual: a Reinforcing slide downhill.",
     build: driftToLowPerformance,
   },
+  {
+    title: "Overshoot and collapse",
+    blurb: "A growth engine burns a finite Resource: it peaks, then crashes.",
+    build: overshootAndCollapse,
+  },
 ]

src/model/types.ts

@@ -76,6 +76,8 @@ export interface StockNode extends BaseNode {
   name: string
   /** Initial accumulated quantity. Optional in the diagram phase; the simulator reads it. */
   initialValue?: number
+  /** Unit of the quantity, e.g. "°C", "people", "$" — shown beside the value (display only). */
+  unit?: string
 }
 
 /**

src/store/model.ts

@@ -117,6 +117,17 @@ export const useModelStore = defineStore("model", () => {
     else node.initialValue = value
   }
 
+  /** Set (or clear) a Stock's display unit (e.g. "°C"). Empty clears it. No-op when unchanged. */
+  function setUnit(id: string, unit: string): void {
+    const node = findNode(id)
+    if (!node || node.kind !== "stock") return
+    const next = unit.trim() || undefined
+    if (node.unit === next) return
+    record()
+    if (next === undefined) delete node.unit
+    else node.unit = next
+  }
+
   /**
    * Set (or clear) how a Flow's rate or a Converter's value is computed (ADR-0004:
    * one of the fixed rules, never a formula). No-op when unchanged.
@@ -279,6 +290,7 @@ export const useModelStore = defineStore("model", () => {
     moveNode,
     renameNode,
     setInitialValue,
+    setUnit,
     setRule,
     setSimSpec,
     removeNode,

src/style.css

@@ -9,7 +9,16 @@
 @plugin "daisyui/theme" {
   name: "light";
   default: true;
-  --color-primary: #16a34a;
+  --color-primary: #ff9ff3;
+}
+
+/* DaisyUI's default (no data-theme) resolves to its *dark* theme via
+   prefers-color-scheme, with a selector (`:root:not([data-theme])`) that outranks
+   a custom light theme's `:where(:root)` — so overriding only the light theme
+   never reached the buttons. Force the brand primary across every theme. */
+:root {
+  --color-primary: #ff9ff3 !important;
+  --color-primary-content: #3d1f3a !important;
 }
 
 @theme {