[BUG] Child task view can regress to Home when stale state snapshot completes late #1456

Problem

During a subtask transition, the webview can render a ready child task and then regress to the Home/Recent Tasks screen when an older empty-message state snapshot completes later. Multiple overlapping publications can appear as an automatic Home/child loop.

Root cause

ClineProvider.postStateToWebview() and postStateToWebviewWithoutTaskHistory() increment clineMessagesSeq only after awaiting getStateToPostToWebview().

Snapshot construction is asynchronous. A request that captured the child while clineMessages was empty can finish after a newer ready-child request. The stale request then receives the higher sequence number, so the webview's stale-state guard accepts the empty messages as newer.

Child startup is particularly exposed because Task.startTask() intentionally clears clineMessages, posts state, and only then adds the child's first message.

Evidence

TLA+ TLC, Alloy 6, and SPIN independently produced the same counterexample:

1. Capture child state with empty messages.
2. Child publishes its first message.
3. Deliver the ready-child snapshot.
4. Deliver the older empty snapshot with the higher completion-order sequence.
5. Webview accepts it and renders Home while the child remains current.

A focused Vitest reproduction against the production methods confirmed the ordering: the newer ready snapshot posted first with sequence 1, then the older empty snapshot posted with sequence 2.

Expected result

Message-state sequence numbers represent publication initiation order, or message-bearing publications are serialized, so a delayed older snapshot cannot replace newer child messages.

Acceptance criteria

• Add a regression test that delays an older empty snapshot while a newer ready-child snapshot posts.
• Verify the delayed snapshot has an older sequence and cannot replace the ready child messages.
• Preserve existing state-post await semantics and non-message state merging.
• Run the full Zoo Code test suite.

Related

• Parent story: [Story 3.2d] Webview-side task-scoping guard + per-task seq #371
• Adjacent extension-side task scoping: [Story 3.2c] Extension-side task-scoping guard #370
• Large message snapshots that increase timing pressure: feat(webview): incremental clineMessages delivery for focused task #630
• Discord report: https://discord.com/channels/1497384592494297201/1513140102929453096/1543686859199094844