feat(editor): add shared loop-highlight state

src/composables/useLoopHighlight.ts

@@ -0,0 +1,48 @@
+/**
+ * Shared loop-hover highlight (C11). Deliberately *not* in the Model store: this
+ * is transient view state that must never be snapshotted for undo or picked up
+ * by autosave (F7). The badge layer sets it on hover; nodes and Information Link
+ * edges read it to light up the loop they belong to.
+ *
+ * A module-level ref makes it a singleton shared by every component that calls
+ * the composable, without prop-drilling through Vue Flow's render tree.
+ */
+import { computed, type Ref, ref } from "vue"
+import type { Loop, LoopType } from "@/model/loops"
+
+interface LoopHighlight {
+  nodes: Set<string>
+  /** Directed `source>target` pairs of the loop's edges, for link highlighting. */
+  edges: Set<string>
+  type: LoopType
+}
+
+const highlighted = ref<LoopHighlight | null>(null)
+
+export function useLoopHighlight(): {
+  highlighted: Ref<LoopHighlight | null>
+  highlight: (loop: Loop) => void
+  clear: () => void
+} {
+  function highlight(loop: Loop): void {
+    const ids = loop.nodeIds
+    const edges = new Set<string>()
+    for (let i = 0; i < ids.length; i++) edges.add(`${ids[i]}>${ids[(i + 1) % ids.length]}`)
+    highlighted.value = { nodes: new Set(ids), edges, type: loop.type }
+  }
+  function clear(): void {
+    highlighted.value = null
+  }
+  return { highlighted, highlight, clear }
+}
+
+/** Ring class for a node when it belongs to the hovered loop (empty otherwise). */
+export function useNodeLoopRing(nodeId: string): Ref<string> {
+  return computed(() => {
+    const hl = highlighted.value
+    if (!hl || !hl.nodes.has(nodeId)) return ""
+    return hl.type === "R"
+      ? "ring-2 ring-success ring-offset-2"
+      : "ring-2 ring-warning ring-offset-2"
+  })
+}