doReconnectAndCatchUp() now uses the replicaFlushedLSN returned by
the reconnect handshake as the catch-up start point, not the
shipper's stale cached value. The replica may have less durable
progress than the shipper last knew.
ReplicaReceiver initialization: flushedLSN now set from the
volume's checkpoint LSN (durable by definition), not nextLSN
(which includes unflushed entries). receivedLSN still uses
nextLSN-1 since those entries are in the WAL buffer even if
not yet synced.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>