feat(model): guard connections to preserve valid structure

src/model/validation.ts

@@ -0,0 +1,51 @@
+/**
+ * Structure guard (C7, F4) — decides what dragging source→target *means* and
+ * whether it's allowed, from the domain rules in CONTEXT.md / ADR-0001:
+ *
+ *   - A Flow terminates on a Stock or Cloud; its other end is a Stock or Cloud
+ *     too (never cloud→cloud, which would bound nothing).
+ *   - An Information Link terminates on a Flow or Converter, never a Stock
+ *     (Stocks change only via Flows). Its source is a Stock, Flow, or Converter,
+ *     never a Cloud (a boundary carries no value).
+ *
+ * The *target* kind disambiguates the gesture: dropping on a Stock/Cloud can only
+ * be a Flow; dropping on a Flow/Converter can only be an Information Link. This
+ * is "guide, don't nag" (F4): the editor refuses an invalid drop mid-gesture
+ * rather than erroring after the fact.
+ */
+import type { Model, ModelNode } from "./types"
+
+/** What a source→target connection would create, or null if the rules forbid it. */
+export type ConnectionIntent = "flow" | "info" | null
+
+export function intentFor(source: ModelNode, target: ModelNode): ConnectionIntent {
+  if (source.id === target.id) return null
+
+  if (target.kind === "stock" || target.kind === "cloud") {
+    // A Flow. Both ends must be Stocks or Clouds, and not cloud→cloud.
+    if (source.kind !== "stock" && source.kind !== "cloud") return null
+    if (source.kind === "cloud" && target.kind === "cloud") return null
+    return "flow"
+  }
+
+  // target is a Flow or Converter → an Information Link.
+  if (source.kind === "cloud") return null
+  return "info"
+}
+
+/** Whether source→target is allowed *and* not a duplicate of an existing edge. */
+export function canConnect(model: Model, sourceId: string, targetId: string): boolean {
+  const source = model.nodes.find((node) => node.id === sourceId)
+  const target = model.nodes.find((node) => node.id === targetId)
+  if (!source || !target) return false
+
+  const intent = intentFor(source, target)
+  if (intent === null) return false
+
+  if (intent === "info") {
+    return !model.infoLinks.some((link) => link.source === sourceId && link.target === targetId)
+  }
+  return !model.nodes.some(
+    (node) => node.kind === "flow" && node.source === sourceId && node.target === targetId,
+  )
+}