[Epic] Transaction/write atomicity & optimistic-concurrency correctness

Tracking epic for transaction-boundary correctness bugs: ifVersion conflict-detection leaks, multi-key reads not atomic to concurrent readers, force-committed over-time transactions leaving orphaned index entries, and fire-and-forget writes escaping the request transaction.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

No files, tests, or entry points are named. Start by examining the transaction boundaries and separating the four listed correctness cases: ifVersion conflict detection, atomic multi-key reads, orphaned index entries, and fire-and-forget writes. Done would require resolving and validating each case as part of this tracking epic.

Written by the indexing model from the issue text.