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/websocket/tree-socket-reducers.test.ts

@@ -81,6 +81,38 @@ describe("applyMoveTreeNode", () => {
     ]);
   });
 
+  it("does NOT create a partial child list when the destination is loaded-but-collapsed (children unloaded) — keeps it lazy-loadable (#159)", () => {
+    // `dstCollapsed` is in the tree but its children were never lazy-loaded
+    // (children === undefined). The OLD behavior inserted `src` as the ONLY
+    // child ([src]), which defeated the lazy-load gate and HID the parent's
+    // other real children. Now the move leaves children unloaded (so expanding
+    // fetches the FULL set, including src) and just flags hasChildren.
+    const tree: SpaceTreeNode[] = [
+      node("dstCollapsed", {
+        position: "a0",
+        hasChildren: false,
+        children: undefined as unknown as SpaceTreeNode[],
+      }),
+      node("src", { position: "a9" }),
+    ];
+    const next = applyMoveTreeNode(tree, {
+      id: "src",
+      parentId: "dstCollapsed",
+      oldParentId: null,
+      index: 0,
+      position: "a4",
+      pageData: {},
+    });
+    const dst = treeModel.find(next, "dstCollapsed");
+    // Children stay unloaded -> the lazy-load gate fetches the FULL set (incl.
+    // src) on expand, rather than showing a misleading partial [src] list.
+    expect(dst?.children).toBeUndefined();
+    expect(dst?.hasChildren).toBe(true);
+    // src moved away from its old root slot (it lives under dstCollapsed
+    // server-side and reappears when the parent is expanded/loaded).
+    expect(next.map((n) => n.id)).not.toContain("src");
+  });
+
   it("flips the OLD parent's hasChildren to false when it is left childless", () => {
     // src is the only child of `old`; moving it to `dst` empties `old`.
     const tree: SpaceTreeNode[] = [
@@ -164,7 +196,9 @@ describe("applyDeleteTreeNode", () => {
             position: "a1",
             parentPageId: "p",
             hasChildren: true,
-            children: [node("grandchild", { position: "a1", parentPageId: "child" })],
+            children: [
+              node("grandchild", { position: "a1", parentPageId: "child" }),
+            ],
           }),
         ],
       }),