diff --git a/sw-block/engine/replication/integration_test.go b/sw-block/engine/replication/integration_test.go
new file mode 100644
index 000000000..1fc304f85
--- /dev/null
+++ b/sw-block/engine/replication/integration_test.go
@@ -0,0 +1,323 @@
+package replication
+
+import (
+	"fmt"
+	"testing"
+)
+
+// ============================================================
+// Phase 05 Slice 4: Integration closure
+//
+// Tests validate V2-boundary cases through the real engine entry
+// path (assignment intent → recovery → completion/escalation),
+// with observability verification.
+// ============================================================
+
+// --- V2 Boundary 1: Changed-address recovery through assignment ---
+
+func TestIntegration_ChangedAddress_FullFlow(t *testing.T) {
+	log := NewRecoveryLog()
+	r := NewRegistry()
+	primary := RetainedHistory{
+		HeadLSN: 100, TailLSN: 30, CommittedLSN: 100,
+	}
+
+	// Initial assignment.
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "vol1-replica1", Endpoint: Endpoint{DataAddr: "10.0.0.1:9333", CtrlAddr: "10.0.0.1:9334", Version: 1}},
+		},
+		Epoch:           1,
+		RecoveryTargets: map[string]SessionKind{"vol1-replica1": SessionCatchUp},
+	})
+
+	s := r.Sender("vol1-replica1")
+	id := s.SessionID()
+	s.BeginConnect(id)
+	log.Record("vol1-replica1", id, "connect", "initial")
+
+	outcome, proof, _ := s.RecordHandshakeFromHistory(id, 80, &primary)
+	log.Record("vol1-replica1", id, "handshake", fmt.Sprintf("outcome=%s proof=%s", outcome, proof.Reason))
+
+	s.BeginCatchUp(id)
+	s.RecordCatchUpProgress(id, 100)
+	s.CompleteSessionByID(id)
+	log.Record("vol1-replica1", id, "completed", "in_sync")
+
+	if s.State() != StateInSync {
+		t.Fatalf("state=%s", s.State())
+	}
+
+	// Replica restarts on new address — assignment with updated endpoint.
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "vol1-replica1", Endpoint: Endpoint{DataAddr: "10.0.0.2:9333", CtrlAddr: "10.0.0.2:9334", Version: 2}},
+		},
+		Epoch:           1,
+		RecoveryTargets: map[string]SessionKind{"vol1-replica1": SessionCatchUp},
+	})
+
+	// Sender identity preserved.
+	if r.Sender("vol1-replica1") != s {
+		t.Fatal("sender identity must survive address change")
+	}
+
+	// New session on new endpoint.
+	id2 := s.SessionID()
+	if id2 == id {
+		t.Fatal("should have new session ID")
+	}
+	if s.Endpoint().DataAddr != "10.0.0.2:9333" {
+		t.Fatalf("endpoint not updated: %s", s.Endpoint().DataAddr)
+	}
+
+	s.BeginConnect(id2)
+	log.Record("vol1-replica1", id2, "connect", "after address change")
+
+	o2, _, _ := s.RecordHandshakeFromHistory(id2, 100, &primary)
+	if o2 != OutcomeZeroGap {
+		t.Fatalf("o2=%s", o2)
+	}
+	s.CompleteSessionByID(id2)
+	log.Record("vol1-replica1", id2, "completed", "zero_gap after address change")
+
+	// Observability.
+	events := log.EventsFor("vol1-replica1")
+	if len(events) < 4 {
+		t.Fatalf("expected at least 4 events, got %d", len(events))
+	}
+	t.Logf("changed-address: %d recovery events logged", len(events))
+}
+
+// --- V2 Boundary 2: NeedsRebuild → rebuild through assignment ---
+
+func TestIntegration_NeedsRebuild_ThenRebuildAssignment(t *testing.T) {
+	r := NewRegistry()
+	primary := RetainedHistory{
+		HeadLSN: 100, TailLSN: 60, CommittedLSN: 100,
+		CheckpointLSN: 40, CheckpointTrusted: true,
+	}
+
+	// Initial catch-up attempt fails — gap beyond retention.
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "10.0.0.1:9333", Version: 1}},
+		},
+		Epoch:           1,
+		RecoveryTargets: map[string]SessionKind{"r1": SessionCatchUp},
+	})
+
+	s := r.Sender("r1")
+	id := s.SessionID()
+	s.BeginConnect(id)
+	o, _, _ := s.RecordHandshakeFromHistory(id, 30, &primary)
+	if o != OutcomeNeedsRebuild {
+		t.Fatalf("should need rebuild: %s", o)
+	}
+
+	// Registry status shows NeedsRebuild.
+	status := r.Status()
+	if status.Rebuilding != 1 {
+		t.Fatalf("rebuilding=%d", status.Rebuilding)
+	}
+
+	// Rebuild assignment from coordinator.
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "10.0.0.1:9333", Version: 1}},
+		},
+		Epoch:           1,
+		RecoveryTargets: map[string]SessionKind{"r1": SessionRebuild},
+	})
+
+	id2 := s.SessionID()
+	s.BeginConnect(id2)
+	s.RecordHandshake(id2, 0, 100)
+
+	// History-driven rebuild source: checkpoint at 40 but tail at 60 →
+	// CheckpointLSN (40) < TailLSN (60) → unreplayable → full base.
+	s.SelectRebuildFromHistory(id2, &primary)
+	s.BeginRebuildTransfer(id2)
+	s.RecordRebuildTransferProgress(id2, 100)
+	s.CompleteRebuild(id2)
+
+	if s.State() != StateInSync {
+		t.Fatalf("state=%s", s.State())
+	}
+
+	status = r.Status()
+	if status.InSync != 1 {
+		t.Fatalf("in_sync=%d", status.InSync)
+	}
+}
+
+// --- V2 Boundary 3: Epoch bump during recovery → new assignment ---
+
+func TestIntegration_EpochBump_DuringRecovery(t *testing.T) {
+	r := NewRegistry()
+	primary := RetainedHistory{HeadLSN: 100, TailLSN: 0, CommittedLSN: 100}
+
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "r1:9333", Version: 1}},
+		},
+		Epoch:           1,
+		RecoveryTargets: map[string]SessionKind{"r1": SessionCatchUp},
+	})
+
+	s := r.Sender("r1")
+	id := s.SessionID()
+	s.BeginConnect(id)
+
+	// Epoch bumps (failover) mid-recovery.
+	r.InvalidateEpoch(2)
+	s.UpdateEpoch(2)
+
+	// Old session dead.
+	if err := s.RecordHandshake(id, 0, 100); err == nil {
+		t.Fatal("old session should be rejected after epoch bump")
+	}
+
+	// New assignment at epoch 2.
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "r1:9333", Version: 1}},
+		},
+		Epoch:           2,
+		RecoveryTargets: map[string]SessionKind{"r1": SessionCatchUp},
+	})
+
+	id2 := s.SessionID()
+	s.BeginConnect(id2)
+	o, _, _ := s.RecordHandshakeFromHistory(id2, 100, &primary)
+	if o != OutcomeZeroGap {
+		t.Fatalf("epoch 2: %s", o)
+	}
+	s.CompleteSessionByID(id2)
+
+	if s.State() != StateInSync {
+		t.Fatalf("state=%s", s.State())
+	}
+}
+
+// --- V2 Boundary 4: Multi-replica mixed outcomes ---
+
+func TestIntegration_MultiReplica_MixedOutcomes(t *testing.T) {
+	r := NewRegistry()
+	primary := RetainedHistory{
+		HeadLSN: 100, TailLSN: 40, CommittedLSN: 100,
+		CheckpointLSN: 50, CheckpointTrusted: true,
+	}
+
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "r1:9333", Version: 1}},
+			{ReplicaID: "r2", Endpoint: Endpoint{DataAddr: "r2:9333", Version: 1}},
+			{ReplicaID: "r3", Endpoint: Endpoint{DataAddr: "r3:9333", Version: 1}},
+		},
+		Epoch: 1,
+		RecoveryTargets: map[string]SessionKind{
+			"r1": SessionCatchUp,
+			"r2": SessionCatchUp,
+			"r3": SessionCatchUp,
+		},
+	})
+
+	// r1: zero-gap.
+	r1 := r.Sender("r1")
+	id1 := r1.SessionID()
+	r1.BeginConnect(id1)
+	o1, _, _ := r1.RecordHandshakeFromHistory(id1, 100, &primary)
+	if o1 != OutcomeZeroGap {
+		t.Fatalf("r1: %s", o1)
+	}
+	r1.CompleteSessionByID(id1)
+
+	// r2: catch-up.
+	r2 := r.Sender("r2")
+	id2 := r2.SessionID()
+	r2.BeginConnect(id2)
+	o2, p2, _ := r2.RecordHandshakeFromHistory(id2, 60, &primary)
+	if o2 != OutcomeCatchUp || !p2.Recoverable {
+		t.Fatalf("r2: outcome=%s proof=%v", o2, p2)
+	}
+	r2.BeginCatchUp(id2)
+	r2.RecordCatchUpProgress(id2, 100)
+	r2.CompleteSessionByID(id2)
+
+	// r3: needs rebuild.
+	r3 := r.Sender("r3")
+	id3 := r3.SessionID()
+	r3.BeginConnect(id3)
+	o3, p3, _ := r3.RecordHandshakeFromHistory(id3, 20, &primary)
+	if o3 != OutcomeNeedsRebuild || p3.Recoverable {
+		t.Fatalf("r3: outcome=%s proof=%v", o3, p3)
+	}
+
+	// Registry status.
+	status := r.Status()
+	if status.InSync != 2 {
+		t.Fatalf("in_sync=%d", status.InSync)
+	}
+	if status.Rebuilding != 1 {
+		t.Fatalf("rebuilding=%d", status.Rebuilding)
+	}
+	if status.TotalCount != 3 {
+		t.Fatalf("total=%d", status.TotalCount)
+	}
+}
+
+// --- Observability ---
+
+func TestIntegration_RegistryStatus_Snapshot(t *testing.T) {
+	r := NewRegistry()
+	r.ApplyAssignment(AssignmentIntent{
+		Replicas: []ReplicaAssignment{
+			{ReplicaID: "r1", Endpoint: Endpoint{DataAddr: "r1:9333", Version: 1}},
+			{ReplicaID: "r2", Endpoint: Endpoint{DataAddr: "r2:9333", Version: 1}},
+		},
+		Epoch: 1,
+	})
+
+	status := r.Status()
+	if status.TotalCount != 2 {
+		t.Fatalf("total=%d", status.TotalCount)
+	}
+	if len(status.Senders) != 2 {
+		t.Fatalf("senders=%d", len(status.Senders))
+	}
+
+	// Both disconnected (no recovery started).
+	for _, ss := range status.Senders {
+		if ss.State != StateDisconnected {
+			t.Fatalf("%s: state=%s", ss.ReplicaID, ss.State)
+		}
+		if ss.Session != nil {
+			t.Fatalf("%s: should have no session", ss.ReplicaID)
+		}
+	}
+}
+
+func TestIntegration_RecoveryLog(t *testing.T) {
+	log := NewRecoveryLog()
+
+	log.Record("r1", 1, "connect", "initial")
+	log.Record("r1", 1, "handshake", "catch-up")
+	log.Record("r2", 2, "connect", "rebuild")
+	log.Record("r1", 1, "completed", "in_sync")
+
+	all := log.Events()
+	if len(all) != 4 {
+		t.Fatalf("events=%d", len(all))
+	}
+
+	r1Events := log.EventsFor("r1")
+	if len(r1Events) != 3 {
+		t.Fatalf("r1 events=%d", len(r1Events))
+	}
+
+	r2Events := log.EventsFor("r2")
+	if len(r2Events) != 1 {
+		t.Fatalf("r2 events=%d", len(r2Events))
+	}
+}
diff --git a/sw-block/engine/replication/observe.go b/sw-block/engine/replication/observe.go
new file mode 100644
index 000000000..551e6ca86
--- /dev/null
+++ b/sw-block/engine/replication/observe.go
@@ -0,0 +1,96 @@
+package replication
+
+// SenderStatus is a read-only observability snapshot of one sender.
+type SenderStatus struct {
+	ReplicaID string
+	Endpoint  Endpoint
+	Epoch     uint64
+	State     ReplicaState
+	Stopped   bool
+	Session   *SessionSnapshot // nil if no session
+}
+
+// RegistryStatus is a read-only observability snapshot of the entire registry.
+type RegistryStatus struct {
+	Senders    []SenderStatus
+	TotalCount int
+	InSync     int
+	Recovering int
+	Degraded   int
+	Rebuilding int
+}
+
+// Status returns a full observability snapshot of the registry.
+func (r *Registry) Status() RegistryStatus {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	status := RegistryStatus{TotalCount: len(r.senders)}
+	for _, s := range r.All() {
+		ss := SenderStatus{
+			ReplicaID: s.ReplicaID(),
+			Endpoint:  s.Endpoint(),
+			Epoch:     s.Epoch(),
+			State:     s.State(),
+			Stopped:   s.Stopped(),
+			Session:   s.SessionSnapshot(),
+		}
+		status.Senders = append(status.Senders, ss)
+
+		switch ss.State {
+		case StateInSync:
+			status.InSync++
+		case StateCatchingUp, StateConnecting:
+			status.Recovering++
+		case StateDegraded:
+			status.Degraded++
+		case StateNeedsRebuild:
+			status.Rebuilding++
+		}
+	}
+	return status
+}
+
+// RecoveryEvent records a significant recovery lifecycle event for debugging.
+type RecoveryEvent struct {
+	ReplicaID string
+	SessionID uint64
+	Event     string
+	Detail    string
+}
+
+// RecoveryLog collects recovery events for observability.
+type RecoveryLog struct {
+	events []RecoveryEvent
+}
+
+// NewRecoveryLog creates an empty recovery log.
+func NewRecoveryLog() *RecoveryLog {
+	return &RecoveryLog{}
+}
+
+// Record adds an event to the log.
+func (rl *RecoveryLog) Record(replicaID string, sessionID uint64, event, detail string) {
+	rl.events = append(rl.events, RecoveryEvent{
+		ReplicaID: replicaID,
+		SessionID: sessionID,
+		Event:     event,
+		Detail:    detail,
+	})
+}
+
+// Events returns all recorded events.
+func (rl *RecoveryLog) Events() []RecoveryEvent {
+	return rl.events
+}
+
+// EventsFor returns events for a specific replica.
+func (rl *RecoveryLog) EventsFor(replicaID string) []RecoveryEvent {
+	var out []RecoveryEvent
+	for _, e := range rl.events {
+		if e.ReplicaID == replicaID {
+			out = append(out, e)
+		}
+	}
+	return out
+}