fix: Batch 3 wiring — production path uses runtime helpers, legacy isolated

H wiring: block_recovery.go now uses runtime.PendingCoordinator
- Removed local pendingRecoveryExecution type + store/take/peek/has/cancel
- ExecutePendingCatchUp/Rebuild delegate to coord.TakeCatchUp/TakeRebuild
- Shutdown uses coord.CancelAll
- Added CancelAll to PendingCoordinator

I wiring: executeCatchUpPlan/executeRebuildPlan replaced
- ExecutePendingCatchUp now calls rt.ExecuteCatchUpPlan with RecoveryManager
  as RecoveryCallbacks (OnCatchUpCompleted/OnRebuildCompleted)
- ExecutePendingRebuild follows same pattern
- Local executeCatchUpPlan/executeRebuildPlan methods removed

J structural: legacy no-core branches extracted
- executeLegacyCatchUp: wraps rt.ExecuteCatchUpPlan for v2Core==nil path
- executeLegacyRebuild: wraps rt.ExecuteRebuildPlan for v2Core==nil path
- Clear "LEGACY NO-CORE COMPATIBILITY" section with structural separation
- runCatchUp/runRebuild now branch cleanly: legacy helper vs core coordinator

Test updates: pendingRecoveryExecution → rt.PendingExecution, field casing,
Plan type assertions.

Validation: all P4, P16B, and ApplyAssignments tests pass.

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

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

@@ -137,3 +137,20 @@ func (pc *PendingCoordinator) Peek(volumeID string) *PendingExecution {
 	defer pc.mu.Unlock()
 	return pc.pending[volumeID]
 }
+
+// CancelAll cancels and removes all pending executions.
+func (pc *PendingCoordinator) CancelAll(reason string) {
+	pc.mu.Lock()
+	all := make(map[string]*PendingExecution, len(pc.pending))
+	for k, v := range pc.pending {
+		all[k] = v
+	}
+	pc.pending = make(map[string]*PendingExecution)
+	pc.mu.Unlock()
+
+	if pc.cancelFn != nil {
+		for _, pe := range all {
+			pc.cancelFn(pe, reason)
+		}
+	}
+}