fix: true mid-executor invalidation test via OnStep hook

CatchUpExecutor.OnStep: optional callback fired between executor-managed
progress steps. Enables deterministic fault injection (epoch bump)
between steps without racing or manual sender calls.

E2_EpochBump_MidExecutorLoop:
- Executor runs 5 progress steps
- OnStep hook bumps epoch after step 1 (after 2 successful steps)
- Executor's own loop detects invalidation at step 2's check
- Resources released by executor's release path (not manual cancel)
- Log shows session_invalidated + exec_resources_released

This closes the remaining FC2 gap: invalidation is now detected
and cleaned up by the executor itself, not by external code.

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

sw-block/engine/replication/executor.go

@@ -16,6 +16,11 @@ type CatchUpExecutor struct {
 	replicaID string
 	sessID    uint64
 	released  bool
+
+	// OnStep is an optional callback invoked between executor-managed steps.
+	// Used for deterministic fault injection in tests (e.g., epoch bump).
+	// step is the 0-based index of the completed step.
+	OnStep func(step int)
 }
 
 // NewCatchUpExecutor creates an executor from a plan. The plan's resources
@@ -68,6 +73,11 @@ func (e *CatchUpExecutor) Execute(progressLSNs []uint64, startTick uint64) error
 			return err
 		}
 
+		// Fire step callback (test hook for fault injection).
+		if e.OnStep != nil {
+			e.OnStep(i)
+		}
+
 		// Check budget after each step.
 		v, err := s.CheckBudget(e.sessID, tick)
 		if err != nil {