Harden semantic boundaries and repair dialogue state

Replace nearest-neighbour personal routing with a frozen class-balanced linear head measured on historical, stratified, cross-validation, holdout, and fresh challenge gates (V-702). Close the four repair handoff holes, preserve nested clarification flows, and route Russian possession statements through structural grammar rather than lexical exceptions (V-573). Owner explicitly requested direct commits to master.

internal/dialogue/stack_test.go

@@ -62,6 +62,22 @@ func TestStackPoppedEntryIsGone(t *testing.T) {
 	}
 }
 
+func TestCompleteTopKeepsSuspendedFlow(t *testing.T) {
+	s := NewClarifyStore(time.Minute)
+	bottom := parked("напомни", pendingBase)
+	top := parked("погода", pendingBase)
+	s.Push("voice", bottom)
+	s.Push("voice", top)
+
+	completed, resumed := s.CompleteTop("voice", pendingBase)
+	if completed != top || resumed != bottom {
+		t.Fatalf("completed=%p resumed=%p, want top=%p bottom=%p", completed, resumed, top, bottom)
+	}
+	if got := s.Depth("voice"); got != 1 {
+		t.Fatalf("depth=%d, want one surviving flow", got)
+	}
+}
+
 // Past MaxStackDepth the oldest entry comes back to the caller instead of
 // vanishing — it is the caller's job to say it was dropped.
 func TestStackDepthBoundReturnsTheDroppedEntry(t *testing.T) {