fix(tree): refresh loaded branches on reconnect so they don't go stale (#159)

Third tree-sync finding (#8). On a socket reconnect after a missed-events gap
(laptop sleep / wifi blip), the resync only invalidated the ROOT sidebar query;
a move/rename/delete that happened INSIDE an already-loaded, expanded branch was
never reflected — the branch stayed stale until the user manually interacted.
(The #2 fix reconciles the root level; this covers the deeper loaded branches.)

- `treeModel.reconcileChildren(tree, parentId, fresh)`: replace a loaded
  branch's DIRECT children with the authoritative fresh set (drop removed, add
  new, reorder to server) while PRESERVING each surviving child's already-loaded
  grandchildren, so deeper expansion is not collapsed. An unloaded branch
  (children === undefined) is left untouched (lazy-load fetches it fresh).
- `loadedOpenBranchIds(tree, openIds)`: the branches a reconnect should refresh
  (open AND loaded). `fetchAllAncestorChildren(..., { fresh: true })` bypasses
  the 30-min sidebar cache so the reconcile sees current data (handler-order
  independent).
- space-tree: on socket `connect`, re-fetch + reconcile each open loaded branch
  of the active space (space-switch-guarded; an unloaded branch is skipped).

Tests: reconcileChildren (drop/add/reorder + preserve grandchildren + unloaded
no-op) and loadedOpenBranchIds (open+loaded only, skip unloaded, nested). The
pure logic is unit-tested; the live socket-reconnect round-trip is not
browser-automated (simulating a reconnect gap is impractical) — sidebar render +
expand were smoke-tested with no regression.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/page/tree/model/tree-model.ts

@@ -223,6 +223,48 @@ export const treeModel = {
     return touched ? out : tree;
   },
 
+  // Replace a parent's DIRECT children with the authoritative `fresh` set while
+  // PRESERVING each surviving child's already-loaded grandchildren (deeper
+  // expansion). Unlike `appendChildren` (add-only), this DROPS children that are
+  // no longer present and reorders to `fresh` — so a move/delete/rename that
+  // happened inside a loaded branch while events were missed (a socket reconnect
+  // gap) is reflected, not left stale (#159 #8). Only used to reconcile an
+  // already-loaded branch against a fresh fetch; a parent with no loaded children
+  // (`children === undefined`) is left untouched (lazy-load handles it).
+  reconcileChildren<T extends object>(
+    tree: TreeNode<T>[],
+    parentId: string,
+    fresh: TreeNode<T>[],
+  ): TreeNode<T>[] {
+    let touched = false;
+    const walk = (nodes: TreeNode<T>[]): TreeNode<T>[] =>
+      nodes.map((n) => {
+        if (n.id === parentId) {
+          // Only reconcile a branch whose children were actually loaded; an
+          // unloaded parent stays unloaded (lazy-load fetches it fresh later).
+          if (n.children === undefined) return n;
+          const prevById = new Map(n.children.map((c) => [c.id, c]));
+          const merged = fresh.map((f) => {
+            const prev = prevById.get(f.id);
+            // Preserve the surviving child's previously loaded grandchildren so
+            // deeper expansion is not collapsed by the reconcile.
+            return prev?.children !== undefined
+              ? { ...f, children: prev.children }
+              : f;
+          });
+          touched = true;
+          return { ...n, children: merged };
+        }
+        if (n.children) {
+          const next = walk(n.children);
+          if (next !== n.children) return { ...n, children: next };
+        }
+        return n;
+      });
+    const out = walk(tree);
+    return touched ? out : tree;
+  },
+
   place<T extends object>(
     tree: TreeNode<T>[],
     sourceId: string,