feat(editor): give each node and link a description

A free-text "why this element is here" note on Stocks, Flows,
Converters, and Information Links — editable in the Inspector next to
the unit, shown read-only in the gloss card below the generic kind
definition on selection. The Inspector now also opens for Information
Links, which previously had no editing surface.

src/components/GlossPanel.vue

@@ -14,7 +14,9 @@ import { useVueFlow } from "@vue-flow/core"
 import { computed } from "vue"
 import { type GlossEntry, INFO_LINK_GLOSS, NODE_GLOSSARY } from "@/model/glossary"
 import type { EdgeData, NodeData } from "@/model/projection"
+import { useModelStore } from "@/store/model"
 
+const store = useModelStore()
 const { getSelectedNodes, getSelectedEdges } = useVueFlow("meadows")
 
 const gloss = computed<GlossEntry | null>(() => {
@@ -32,6 +34,33 @@ const gloss = computed<GlossEntry | null>(() => {
   }
   return null
 })
+
+/**
+ * The selected element's own "why it's here" note (G4) — distinct from the
+ * generic `gloss` above, which defines the *kind*. Read live from the store so
+ * it tracks edits, and resolves the same selection a pipe/info edge stands in for.
+ */
+const description = computed<string | null>(() => {
+  const nodes = getSelectedNodes.value
+  const edges = getSelectedEdges.value
+
+  if (nodes.length === 1 && edges.length === 0) {
+    const node = store.model.nodes.find((n) => n.id === nodes[0].id)
+    return node && node.kind !== "cloud" ? (node.description ?? null) : null
+  }
+  if (edges.length === 1 && nodes.length === 0) {
+    const edge = edges[0]
+    const kind = (edge.data as EdgeData | undefined)?.kind
+    if (kind === "pipe") {
+      const flow = store.model.nodes.find((n) => n.id === edge.id.split("::")[0])
+      return flow && flow.kind !== "cloud" ? (flow.description ?? null) : null
+    }
+    if (kind === "info") {
+      return store.model.infoLinks.find((l) => l.id === edge.id)?.description ?? null
+    }
+  }
+  return null
+})
 </script>
 
 <template>
@@ -41,5 +70,13 @@ const gloss = computed<GlossEntry | null>(() => {
   >
     <div class="text-sm font-semibold">{{ gloss.term }}</div>
     <p class="mt-0.5 text-xs leading-snug text-base-content/70">{{ gloss.short }}</p>
+    <!-- This element's own rationale, below a divider so it reads apart from the
+         generic definition above (the "tour" note for the loaded sample). -->
+    <p
+      v-if="description"
+      class="mt-2 border-t border-base-300 pt-2 text-xs leading-snug text-base-content/80"
+    >
+      {{ description }}
+    </p>
   </div>
 </template>

src/components/Inspector.vue

@@ -14,20 +14,25 @@
 import { useVueFlow } from "@vue-flow/core"
 import { computed } from "vue"
 import type { EdgeData } from "@/model/projection"
-import type { ConverterNode, FlowNode, Rule, StockNode } from "@/model/types"
+import type { ConverterNode, FlowNode, InformationLink, Rule, StockNode } from "@/model/types"
 import { useModelStore } from "@/store/model"
 
 const store = useModelStore()
 const { getSelectedNodes, getSelectedEdges } = useVueFlow("meadows")
 
-/** The single selected element's id — a node directly, or a Flow via its pipe edge. */
+/**
+ * The single selected element's id — a node directly, a Flow via its pipe edge,
+ * or an Information Link via its info edge (the edge id *is* the link id).
+ */
 const selectedId = computed<string | null>(() => {
   const nodes = getSelectedNodes.value
   const edges = getSelectedEdges.value
   if (nodes.length === 1 && edges.length === 0) return nodes[0].id
   if (edges.length === 1 && nodes.length === 0) {
     const edge = edges[0]
-    if ((edge.data as EdgeData | undefined)?.kind === "pipe") return edge.id.split("::")[0]
+    const kind = (edge.data as EdgeData | undefined)?.kind
+    if (kind === "pipe") return edge.id.split("::")[0]
+    if (kind === "info") return edge.id
   }
   return null
 })
@@ -41,8 +46,21 @@ const element = computed<StockNode | FlowNode | ConverterNode | null>(() => {
   return null
 })
 
+/** The live Information Link behind the selection, when a link (not a node) is selected. */
+const link = computed<InformationLink | null>(() => {
+  const id = selectedId.value
+  if (!id) return null
+  return store.model.infoLinks.find((l) => l.id === id) ?? null
+})
+
 const KIND_LABEL = { stock: "Stock", flow: "Flow", converter: "Converter" } as const
 
+/** A link's endpoints are always named nodes — it never touches a Cloud (validation.ts). */
+function nodeName(id: string): string {
+  const node = store.model.nodes.find((n) => n.id === id)
+  return node && "name" in node ? node.name : ""
+}
+
 /** Every rule carries exactly one number (a value or a factor); read it uniformly. */
 function ruleNumber(rule?: Rule): number {
   if (!rule) return 0
@@ -68,6 +86,13 @@ function onUnit(event: Event): void {
   store.setUnit(el.id, (event.target as HTMLInputElement).value)
 }
 
+/** Write the description for whichever element (node or link) is selected. */
+function onDescription(event: Event): void {
+  const id = element.value?.id ?? link.value?.id
+  if (!id) return
+  store.setDescription(id, (event.target as HTMLTextAreaElement).value)
+}
+
 function onKind(event: Event): void {
   const el = element.value
   if (el?.kind !== "flow" && el?.kind !== "converter") return
@@ -95,9 +120,11 @@ const RULE_HINT: Record<Rule["kind"], string> = {
 
 <template>
   <div
-    v-if="element"
+    v-if="element || link"
     class="absolute top-3 right-3 z-20 w-60 rounded-box border border-base-300 bg-base-100/95 p-3 shadow-md backdrop-blur"
   >
+    <!-- A named node: Stock (value + unit) or Flow/Converter (rule). -->
+    <template v-if="element">
       <div class="flex items-baseline gap-2">
         <span class="text-sm font-semibold">{{ element.name }}</span>
         <span class="text-xs text-base-content/50">{{ KIND_LABEL[element.kind] }}</span>
@@ -160,5 +187,29 @@ const RULE_HINT: Record<Rule["kind"], string> = {
           {{ RULE_HINT[element.rule.kind] }}
         </p>
       </template>
+    </template>
+
+    <!-- An Information Link: its endpoints and polarity, then its description. -->
+    <template v-else-if="link">
+      <div class="flex items-baseline gap-2">
+        <span class="text-sm font-semibold">Information Link</span>
+        <span class="text-xs text-base-content/50">{{ link.polarity === "+" ? "+" : "−" }}</span>
+      </div>
+      <p class="mt-1 text-xs text-base-content/60">
+        {{ nodeName(link.source) }} → {{ nodeName(link.target) }}
+      </p>
+    </template>
+
+    <!-- Shared across nodes and links: the "why this element is here" note (G4). -->
+    <label class="mt-2 block">
+      <span class="text-xs text-base-content/60">Description</span>
+      <textarea
+        rows="3"
+        class="textarea textarea-bordered textarea-sm mt-1 w-full leading-snug"
+        :value="(element ?? link)?.description ?? ''"
+        placeholder="Why this element is here…"
+        @change="onDescription"
+      />
+    </label>
   </div>
 </template>

src/model/io.ts

@@ -99,6 +99,14 @@ function nodeError(value: unknown, index: number): string | null {
   ) {
     return `${at}.rule must be a valid rule (constant/proportional/gap)`
   }
+  // The selection-time rationale (display only); any named node may carry one.
+  if (
+    kind !== "cloud" &&
+    value.description !== undefined &&
+    typeof value.description !== "string"
+  ) {
+    return `${at}.description must be a string`
+  }
   return null
 }
 
@@ -110,6 +118,9 @@ function linkError(value: unknown, index: number): string | null {
   if (typeof value.source !== "string") return `${at}.source must be a string`
   if (typeof value.target !== "string") return `${at}.target must be a string`
   if (!isPolarity(value.polarity)) return `${at}.polarity must be "+" or "-"`
+  if (value.description !== undefined && typeof value.description !== "string") {
+    return `${at}.description must be a string`
+  }
   return null
 }
 

src/model/types.ts

@@ -78,6 +78,8 @@ export interface StockNode extends BaseNode {
   initialValue?: number
   /** Unit of the quantity, e.g. "°C", "people", "$" — shown beside the value (display only). */
   unit?: string
+  /** Why this Stock exists in *this* model — a free-text note surfaced on selection (display only). */
+  description?: string
 }
 
 /**
@@ -95,6 +97,8 @@ export interface FlowNode extends BaseNode {
   target: string
   /** How its rate is computed each instant (ADR-0004). Optional in the diagram phase. */
   rule?: Rule
+  /** Why this Flow exists in *this* model — a free-text note surfaced on selection (display only). */
+  description?: string
 }
 
 /**
@@ -106,6 +110,8 @@ export interface ConverterNode extends BaseNode {
   name: string
   /** How its value is computed each instant (ADR-0004). Optional in the diagram phase. */
   rule?: Rule
+  /** Why this Converter exists in *this* model — a free-text note surfaced on selection (display only). */
+  description?: string
 }
 
 /**
@@ -131,6 +137,8 @@ export interface InformationLink {
   /** Target node id — a Flow or Converter, never a Stock (ADR-0001). */
   target: string
   polarity: Polarity
+  /** Why this link exists in *this* model — a free-text note surfaced on selection (display only). */
+  description?: string
 }
 
 /** One saved document: the unit of save / export / reopen. */

src/store/model.ts

@@ -128,6 +128,28 @@ export const useModelStore = defineStore("model", () => {
     else node.unit = next
   }
 
+  /**
+   * Set (or clear) the free-text description on a named node or an Information
+   * Link — the "why this element exists" note surfaced on selection. Empty
+   * clears it; no-op when unchanged, so a blur with no edit doesn't burn undo.
+   */
+  function setDescription(id: string, text: string): void {
+    const next = text.trim() || undefined
+    const node = findNode(id)
+    if (node && node.kind !== "cloud") {
+      if (node.description === next) return
+      record()
+      if (next === undefined) delete node.description
+      else node.description = next
+      return
+    }
+    const link = model.value.infoLinks.find((l) => l.id === id)
+    if (!link || link.description === next) return
+    record()
+    if (next === undefined) delete link.description
+    else link.description = 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.
@@ -291,6 +313,7 @@ export const useModelStore = defineStore("model", () => {
     renameNode,
     setInitialValue,
     setUnit,
+    setDescription,
     setRule,
     setSimSpec,
     removeNode,