Detect ABA edits during watcher indexing

[lzl51230]

Related to #1113.

An SVN observation can stage working-copy state A, while the indexing pipeline reads and persists intermediate state B. If the working copy returns to A before acknowledgement, the watcher can acknowledge baseline A even though the graph contains B; the next poll then sees A as unchanged and leaves the graph stale.

Tie acknowledgement to the exact indexed input digest, or perform a post-publication observation check and schedule another update when it differs. Add a deterministic A-to-B-to-A concurrency test.

[DeusData]

Tracked as a 0.9.2-rc graph-correctness blocker for #1113/#1127. The acknowledgement must correspond to the content the pipeline actually persisted; otherwise an A-to-B-to-A sequence can permanently bless a stale graph. The deterministic test should control the observation and publication barriers and prove a follow-up update is scheduled whenever the post-publication state differs.