app-server: claim thread teardown groups atomically

[winston-openai]

Summary

Generalize the tracked teardown coordinator from one ThreadId to an exact operation-owned group. Initial claims deduplicate and preflight the complete ID set under one registry lock, while group extensions immediately own free IDs and register an exact weak successor for every contested ID.

A coordinator-wide affine freeze guard serializes tree-discovery and extension phases even when callers must drop the app-server thread-list permit to await an older teardown. When that predecessor releases, the registry transfers each contested ThreadId directly to the live successor before signaling completion. This removes both cross-extension deadlocks and the scheduler-sized admission gap where a descendant could otherwise be resumed or attached.

Operation identity, exact Arc release checks, and cancellation-aware weak handoffs keep stale owners from removing or resurrecting successor claims. Existing single-ID idle unload and cleanup continue through compatibility wrappers backed by the same coordinator.

Stacked on #31399. The next layer uses this primitive for complete-tree teardown and retains the same claim through archive/delete mutation or same-ThreadId successor publication.

Validation

• Coordinator tests cover provisional cancellation, singleflight lifetime, multi-owner conflict deduplication, free-sibling ownership during waits, gapless handoff, freeze serialization, stale successor cancellation, and drain behavior.
• Existing lifecycle reservation and unsubscribe suites pass through the single-ID adapters.
• just fix -p codex-app-server
• just fmt

[winston-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: 1f6c042c0f

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[winston-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: b2c07ba8d8

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