fix(desktop): bind Side Conversation events to Host admission

[Astro-Han]

Summary

• Give every Side Conversation send or steer one admission ticket; Runtime Host remains the sole authority that accepts or retracts it and names the exact owning Turn.
• Bind direct starts from the Host command result. For queued current-Turn and successor-Turn admission, consume only Host-owned queue/root projection facts; unrelated terminal events never infer ownership.
• Buffer pre-admission events until Host resolves the exact ticket, including events arriving while Stop outcome is unknown.
• Preserve unresolved admission across recoverable subscription failure and Host replacement; replacement events follow the current validated Host target epoch.
• Scope admission projection, retry, unknown-outcome, and Stop fencing to canonically labelled Side Conversation Sessions. Ordinary Desktop/CLI streams keep their previous contract.

No durable message schema, settlement/recovery policy, CLI projection, or Desktop primary-conversation projection is changed.

Behavior evidence

The production-shaped React hook + Workbar seam covers:

• old Turn settled, new send waiting for Host admission, then a late old terminal event;
• successor admission and text arriving before the command response and replaying onto only that Turn;
• Host admission preceding the empty successor queue snapshot;
• explicit queue retraction using the actually submitted ticket;
• recoverable subscription failure and Host replacement preserving the original ticket;
• admission-scoped Stop for direct, queued, in-flight, and successor ownership, including an admitted event arriving while Stop outcome remains unknown;
• busy retry identity, fork/copy cleanup, disposal, and observation-seed failure;
• ordinary sends and shared CLI/main-conversation streams remaining unchanged.

Verification

• npm run build
• 77 focused Desktop tests passed across Quote Companion, Runtime Host execution IPC/observer, boundary validation, disposal, and Workbar adapter
• 156 focused Runtime Host protocol/projector/canonical/subscription tests passed
• 36 Runtime event-mapper tests passed
• 85 focused CLI Runtime Host adapter/driver tests passed
• Biome changed-file formatting and git diff --check passed

Exact-head CI: passed for 364dd1a96adaac3ba1bbaecbdb4da453cbcf11f6 (run 32772641137).

AI use

Codex contributed the implementation, regression tests, verification, and adversarial/simplification review. No commit from Draft PR #3633 was cherry-picked or copied; its old branch/review was used only as failure evidence.