refactor(canvas): promote collapsed to first-class NodeData field + edge rewire

Three follow-ups to PR #158 review:

1. Promote collapsed to NodeData.collapsed
   The flag was previously stashed inside NodeData.custom_colors, which is
   a colors/style object — semantically wrong. Move it to a first-class
   boolean on NodeData. Persistence keeps the existing API shape: serialize
   writes it into the custom_colors blob (alongside width/height/z_order,
   matching how groupRect already stashes layout metadata), and deserialize
   hoists it back. Legacy saves from the original PR shape load correctly.

2. Re-route cross-zone edges to the collapsed ancestor
   Previously any edge touching a hidden node was dropped, so a Zigbee
   coordinator outside a collapsed mesh lost all visible links to it.
   rewireEdgesForCollapse now walks each endpoint up the parentId chain to
   its nearest visible ancestor, surfaces a single stub edge on the
   collapsed zone, de-dupes parallel rewires (a 20-device mesh becomes one
   stub, not twenty), and drops edges that would self-loop on a zone or
   reference an orphan.

3. Revert package-lock.json churn
   The 63-line diff from the original PR was npm-version drift (libc
   arrays stripped from optional deps), unrelated to the feature.

Tests:
- canvasStore.collapse: updated to assert on data.collapsed.
- collapseFilter: 8 cases for visibility + 7 for edge rewire, covering
  cross-boundary, nested collapse, sibling self-loop, mesh dedup, and
  orphan endpoints.
- canvasSerializer.collapse: round-trip + legacy-shape compat.

frontend/src/utils/canvasSerializer.ts

@@ -76,6 +76,9 @@ export function serializeNode(n: Node<NodeData>): Record<string, unknown> {
         ...n.data.custom_colors,
         width: n.measured?.width ?? n.width ?? 360,
         height: n.measured?.height ?? n.height ?? 240,
+        // Stash collapse state inside custom_colors so the API/YAML blob does
+        // not need a new column. Hoisted back to `data.collapsed` on load.
+        collapsed: n.data.collapsed ?? false,
       },
     }
   }
@@ -139,11 +142,15 @@ export function deserializeApiNode(
     const w = (n.custom_colors?.width as number | undefined) ?? 360
     const h = (n.custom_colors?.height as number | undefined) ?? 240
     const z = (n.custom_colors?.z_order as number | undefined) ?? 1
+    // Hoist persisted collapse flag from the custom_colors stash to a
+    // first-class field on NodeData. Tolerates legacy saves that already had
+    // it there from before the type was promoted.
+    const collapsed = Boolean(n.custom_colors?.collapsed)
     return {
       id: n.id,
       type: 'groupRect',
       position: { x: n.pos_x, y: n.pos_y },
-      data: n as unknown as NodeData,
+      data: { ...(n as unknown as NodeData), collapsed },
       width: w,
       height: h,
       zIndex: z - 10,