Gate AppServer pre-dispatch cancellation before releasing initialize

[lynnswap]

Problem

AppServerClientTests.backendAdmissionRefusesThreadStartBeforeDispatch creates the cancellation task and immediately opens the held initialize response. It does not wait for ReviewStartAdmission to record the cancellation owner.

If initialization resumes first, the backend legitimately reaches thread/start; the fake has no queued thread response and decodes {} as Thread.Start.Response, producing the intermittent keyNotFound("thread") failure seen in full package runs. The test passes individually when cancellation happens to win.

Required fix

After creating the cancellation task, wait for the semantic cancellation admission (.mcpClient("Stop")) before opening the initialize gate. Keep production unchanged. Search the neighboring AppServer admission fixtures for the same task-scheduling assumption.

Acceptance

• Deterministically proves zero thread/start writes after admitted pre-dispatch cancellation.
• Repeated focused runs and the full AppServer/full package suites pass.
• No sleeps, Task.yield(), request-count ordering, or production changes.

[lynnswap]

A test-only fix is remotely checkpointed at codex/fix-joined-cancellation-test@85cf21d. It gates the intended cancellation owner before releasing initialize, thread-start, and review-start response gates in the three neighboring admission fixtures.

Verification: AppServerClientTests passed 123/123 on the v0.6.2/Wave3A test baseline, and the formerly flaky backendAdmissionRefusesThreadStartBeforeDispatch passed 10 consecutive focused runs. No production files changed. The commit will be integrated into Ready PR #117 before the final full-package gate.