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

[Astro-Han]

Summary

• Give every Side Conversation send/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; the renderer never lets an unrelated terminal event infer ownership.
• Buffer pre-admission events until Host resolves the exact ticket. A late terminal event from the old Turn cannot claim the new send, while successor events that arrive before the command response are replayed.
• Preserve unresolved admission across recoverable subscription failure and Host epoch replacement. outcome_unknown means “still unknown”, not rejection or permission to mint another ticket.
• Scope retry, unknown-outcome, and Stop fencing to Side Conversation only. Ordinary Desktop sends retain their prior one-call, independent-message-id, and error-propagation 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 arriving before the command response;
• recoverable subscription failure and cross-epoch outcome_unknown preserving the original ticket;
• admission-scoped Stop for direct, queued, in-flight, and successor ownership, including unknown Stop outcome;
• busy retry identity, fork/copy cleanup, disposal, and observation-seed failure;
• ordinary sends not entering Side Conversation admission retry semantics;
• shared CLI/main-conversation event streams remaining unchanged.

Verification

• npm run build
• npm --workspace @maka/desktop run typecheck
• 41 focused Desktop tests passed across Quote Companion, Runtime Host execution IPC, 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: pending for f51928a5bc7201870885e6c5a1bc6475e9cfc8e5.

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.