refactor: make bounded recovery command addressing replica-scoped

Replace the remaining volume-scoped recovery command and pending slot
with replica-scoped addressing on the bounded core-present path. This
preserves the current single-replica catch-up and rebuilding behavior
while removing the structural blocker for later multi-replica startup
ownership.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sw-block/engine/replication/command.go

@@ -37,6 +37,7 @@ func (StartRecoveryTaskCommand) commandName() string { return "start_recovery_ta
 
 type StartCatchUpCommand struct {
 	VolumeID  string
+	ReplicaID string
 	TargetLSN uint64
 }
 
@@ -44,6 +45,7 @@ func (StartCatchUpCommand) commandName() string { return "start_catchup" }
 
 type StartRebuildCommand struct {
 	VolumeID  string
+	ReplicaID string
 	TargetLSN uint64
 }