F2: Validate refuses a write or load that would give a key already held by
one home store a second store; the message names the key and both stores and
nothing lands in either store or the cache.
F3: TryCommit enters the cache gate normally; only the store lock attempt is
non-blocking, so Contended means another writer holds the store.
F4: PushAll and Commit are shown to wait on a held <path>.lock; the race test
asserts mismatches occurred and that Task.WaitAll finished in time.
F5: a failed page write leaves the manifest bytes and the previous generation
intact.
Lock order (F1) and commit merge semantics (F6) are unchanged.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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