fix(comments): order shared cache settlements

[Chris0Jeky]

Summary

Hardens the shared per-card comment cache against late reads, overlapping writes, board revisits, same-board refreshes, and session changes:

• only the latest overlapping read for a card may commit;
• successful mutations invalidate reads that began against an older snapshot;
• board-visit generations distinguish A→B→A and logout/login even when the board id repeats;
• same-board detail refreshes may receive a successful in-flight create/update/delete into their newly installed cache;
• successful writes from an older visit trigger a bounded authoritative reconciliation only when that same board is currently reopened;
• same-comment update/delete requests serialize so server commit order follows user intent despite no revision/If-Match field;
• queued writes recheck the initiating visit before transport starts, preventing pre-logout intent from using later-session credentials;
• the first, unqueued write starts transport synchronously;
• create deduplicates by stable comment id.

Tests

Focused regressions cover stale reads after create/update/delete, out-of-order reads, navigation and A→B→A visits, same-board refresh before write settlement, old-visit post-write reconciliation, stable-id deduplication, reverse-order same-comment intents, and cancellation of queued work after session replacement.

Verification

Exact head: 66e725d563e5f7d3582196346bb84e5d0adc87ce

• Smart CI Self-Test run 35531671285: success.
• CI Extended run 35531671492: success.
• Required CI run 35531671472: success across frontend lint/typecheck/build/full tests on Ubuntu and Windows, backend unit/API integration on Ubuntu and Windows, architecture, migrations, containers, dependency/SAST/secret scans, docs governance, worktree contracts and E2E smoke.
• All five Codex findings across the review rounds are addressed with dedicated regressions, replied to, and resolved.
• Fresh exact-head diff review completed; no DTO, route, migration, control-plane or canonical-document changes.

Closes #3303

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-20T21:54:21.082899Z	66e725d	Draft marked ready

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Reviewed commit: cae9a12194

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