fix(tree): stop silent page loss on move-to-unloaded-parent + reconnect ghost roots (#159)

Two confirmed P1 data-loss findings in the sidebar tree sync.

#1 — Move into an unloaded/collapsed parent silently dropped pages. When a
moveTreeNode (or addTreeNode) broadcast targeted a parent whose children were
NOT yet lazy-loaded, `insertByPosition` did `kids = parent.children ?? []` and
inserted the moved node, MATERIALIZING a misleading partial child list
(`[movedNode]`) out of an unloaded (`children === undefined`) parent. The
lazy-load gate fetches only when children are absent/empty, so it then refused
to fetch — leaving the parent showing ONLY the moved node and HIDING all its
other real children (and, when the parent wasn't in the tree at all, the node
was removed and never re-fetched). Fix: `insertByPosition` distinguishes
`children === undefined` (not loaded) from `[]` (loaded-empty) and, for an
unloaded parent, does NOT insert — it leaves children unloaded and just flags
`hasChildren`, so expanding fetches the FULL set (including the moved/added
node) via the existing lazy-load.

#2 — After a socket reconnect, a deleted/moved-away root lingered as a 404
"ghost". `mergeRootTrees` was append-only: it kept every previously-loaded root
and only added new ones, so a root removed during the missed-events gap was
never dropped. It runs only once all root pages are fetched, so the incoming
list is the authoritative complete root set — fix reconciles to it (drop roots
absent from incoming) while PRESERVING each surviving root's lazy-loaded
subtree and refreshing its own fields.

Tests: insertByPosition unloaded-vs-loaded-empty parent; the move reducer
keeps a collapsed destination lazy-loadable instead of partial; mergeRootTrees
drops a ghost root, preserves a surviving subtree, adds new roots, refreshes
fields. The existing "remove when parent not in tree" reducer test still holds.

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

apps/client/src/features/page/tree/utils/utils.ts

@@ -214,21 +214,36 @@ export function appendNodeChildren(
 }
 
 /**
- * Merge root nodes; keep existing ones intact, append new ones,
+ * Reconcile the loaded root nodes to the authoritative INCOMING set (the
+ * server's complete current roots for the space), preserving any lazy-loaded
+ * children/subtree of a root that still exists.
+ *
+ * This runs only once all root pages are fetched, so `incomingRoots` is the full
+ * server root set and is authoritative for WHICH roots exist:
+ *  - a root in BOTH: kept, with its own fields refreshed from `incoming` (so a
+ *    rename/move during a gap shows) while PRESERVING its previously lazy-loaded
+ *    `children` (expanded subtrees + open-state survive a refetch);
+ *  - a root only in `incoming`: a new root, added as-is;
+ *  - a root only in `prev`: it was DELETED or moved under another page while we
+ *    were not receiving events (e.g. a socket reconnect after a sleep/wifi gap).
+ *    It is DROPPED instead of lingering as a 404 "ghost" root (#159 #2). The old
+ *    append-only merge kept it forever.
  */
 export function mergeRootTrees(
   prevRoots: SpaceTreeNode[],
   incomingRoots: SpaceTreeNode[],
 ): SpaceTreeNode[] {
-  const seen = new Set(prevRoots.map((r) => r.id));
+  const prevById = new Map(prevRoots.map((r) => [r.id, r]));
 
-  // add new roots that were not present before
-  const merged = [...prevRoots];
-  incomingRoots.forEach((node) => {
-    if (!seen.has(node.id)) merged.push(node);
+  const reconciled = incomingRoots.map((incoming) => {
+    const prev = prevById.get(incoming.id);
+    // Preserve the previously loaded children/subtree (the root query returns
+    // only top-level roots, so `incoming` carries no children); refresh the
+    // node's own fields from the authoritative incoming copy.
+    return prev ? { ...incoming, children: prev.children } : incoming;
   });
 
-  return sortPositionKeys(merged);
+  return sortPositionKeys(reconciled);
 }
 
 // Collect every node id in the tree (roots, branches, leaves). Used by