bug(sessions): retire replacement when its source placement disappears

[Juliusolsson05]

A read-only orchestration review of #812 reproduced a pre-existing replacement ownership race: close a source pane while replaceSession awaits provider spawn. The successor is inserted into session metadata and returned, but no grid/detached placement remains to remap. The new external lifecycle receipt can consequently report completed for an invisible orphan backend.

Expected: replacement revalidates the source's current durable placement and identity after asynchronous spawn/retirement. If it can no longer commit, retire the successor through the existing owned-backend cleanup and report failure/uncertainty, preserving the user's close/bury/layout action.

Reproduction evidence: production-hook in-memory deferred-spawn/concurrent-close probe from orchestration reviewer 8f6e2b0a-a700-44c6-804f-e76f4d13c038 at 961d542e. Add a deterministic real-hook regression. This is separate from #807's current-draft transfer bug and A5/A6's independently owned fixes.

Refs #795, #812.

[Juliusolsson05]

Two independent Agent Code orchestration reviews completed for the full feature at 961d542eee331f773d2b8b58878a9ef16eb08930, base 5d641845:

• Reviewer 96b42118-6a39-4e98-8faf-1b4501bcde0f: no actionable findings; 6 unit files/14 tests and 9 renderer files/23 tests passed. Noted an extra EOF blank line, now removed.
• Reviewer 8f6e2b0a-a700-44c6-804f-e76f4d13c038: two P2 findings, both reproduced with production-handler probes: (1) source closes during spawn and replacement reports completed with no placement; (2) workspace navigation reports requested target even if focus moves during frame acknowledgment.

Both findings are resolved in 2b0058d7. #815 tracks the pre-existing ownership race. The fix checks the synchronous live store before retirement and at remap commit, cleans an uncommittable successor through the owned-backend boundary, and transfers the draft only after placement succeeds. Navigation now checks canonical effective focus. Three deterministic regressions fail against the reviewed revision and pass corrected; existing draft continuity remains green. Full types, test contract and renderer121files514tests pass.

Both original reviewers are rechecking this final revision. CI must pass on it before the user-authorized merge; no merge has occurred.