feat(editor): give each Stock a unit

A Stock can carry a display unit (°C, people, $, …): a new optional field, an
inspector text input, validation + round-trip in io, and a readout beside the
live value on the canvas. Equip the demonstrative samples (Water L, Balance $,
Coffee °C, Population/Epidemic people, Yeast cells). Also bumps the fill gauge
opacity for a more vivid level.

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/nodes/StockNode.vue

@@ -37,7 +37,7 @@ 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>
@@ -45,7 +45,8 @@ const fill = computed(() => sim.fill(props.id))
     <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) }}
+        {{ formatValue(value)
+        }}<span v-if="stock.unit" class="ml-0.5 text-base-content/50">{{ stock.unit }}</span>
       </div>
     </div>
     <Handle :id="HANDLE_OUT" type="source" :position="Position.Right" />

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

@@ -77,6 +77,7 @@ 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 +109,7 @@ 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 +135,7 @@ 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 +172,7 @@ 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 +210,7 @@ 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 +326,13 @@ 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",

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,