server/storage/mvcc: fix progressIfSync lock-order inversion that deadlocks the apply loop

[lazureykis]

Fixes #22184.

progressIfSync held watchableStore.mu and then acquired store.mu (via s.rev() → store.Read()), the reverse of the order documented on watchableStore.mu ("should never be locked before locking store.mu"). With a concurrent KV().Commit() queuing a writer on store.mu (as the snapshot-send path in createMergedSnapshotMessage does) and an in-flight write txn, this formed a three-way deadlock that wedged the apply loop. Full analysis and a deterministic reproduction are in #22184.

This reads the current revision directly under revMu — the same watchable.mu → revMu ordering already used by watch(), syncWatchers(), and moveVictims() — instead of opening a full read transaction that reaches for store.mu.

Adds TestProgressIfSyncDeadlock, which deadlocks (blocks at a 3s deadline) before this change and passes after. The existing mvcc watch/sync tests remain green under -race.

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AI disclosure: this change was prepared with AI assistance (Claude). I reviewed the analysis, verified the fix and the regression test against the source, and ran the tests myself. The commit carries an Assisted-by: trailer accordingly.

[kubernetes-prow]

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