rebuildFullExtent updated superblock.WALCheckpointLSN but not the
flusher's internal checkpointLSN. NewReplicaReceiver then read
stale 0 from flusher.CheckpointLSN(), causing post-rebuild
flushedLSN to be wrong.
Added Flusher.SetCheckpointLSN() and call it after rebuild
superblock persist. TestRebuild_PostRebuild_FlushedLSN_IsCheckpoint
flips FAIL→PASS.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>