refactor: Batch 4 steps 2+3 — rebuild status port + recovery bundle factory

Step 2: Rebuild completion status port
- New runtime.RebuildCompletionStatus + DeriveRebuildCommitted:
  reusable shaping logic for post-rebuild snapshot → RebuildCommitted event
- block_recovery.go OnRebuildCompleted: delegates to DeriveRebuildCommitted,
  host only reads raw snapshot via readRebuildStatus (thin binding)
- Removed 15 lines of inline flushedLSN/checkpointLSN/achievedLSN computation

Step 3: Recovery bundle factory
- New buildRecoveryBundle: shared host-side setup for both catch-up and rebuild
  (creates Reader + Pinner + StorageAdapter + Executor + RecoveryDriver)
- runCatchUp and runRebuild both use buildRecoveryBundle instead of
  duplicating the WithVolume → NewReader → NewPinner → NewStorageAdapter →
  NewExecutor → RecoveryDriver chain
- runCatchUp/runRebuild are now thin host-shell methods

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

sw-block/engine/replication/runtime/rebuild_status.go

@@ -0,0 +1,35 @@
+package runtime
+
+import engine "github.com/seaweedfs/seaweedfs/sw-block/engine/replication"
+
+// RebuildCompletionStatus holds the post-rebuild state read from the backend.
+// The host adapter fills this after a successful rebuild; the runtime helper
+// uses it to emit the core RebuildCommitted event.
+type RebuildCompletionStatus struct {
+	CommittedLSN  uint64
+	CheckpointLSN uint64
+}
+
+// DeriveRebuildCommitted computes the RebuildCommitted event from
+// post-rebuild status and the original plan. This is the reusable
+// shaping logic — the host only needs to supply the raw snapshot values.
+func DeriveRebuildCommitted(volumeID string, status RebuildCompletionStatus, plan *engine.RecoveryPlan) engine.RebuildCommitted {
+	flushedLSN := status.CommittedLSN
+	if flushedLSN == 0 {
+		flushedLSN = plan.RebuildTargetLSN
+	}
+	checkpointLSN := status.CheckpointLSN
+	if checkpointLSN == 0 {
+		checkpointLSN = plan.RebuildTargetLSN
+	}
+	achievedLSN := flushedLSN
+	if checkpointLSN > achievedLSN {
+		achievedLSN = checkpointLSN
+	}
+	return engine.RebuildCommitted{
+		ID:            volumeID,
+		AchievedLSN:   achievedLSN,
+		FlushedLSN:    flushedLSN,
+		CheckpointLSN: checkpointLSN,
+	}
+}