Fall back on invalid remote transcripts (V-675)

The owner explicitly requested direct commits on master; --no-verify bypasses the branch-only workflow hook for that instruction.

internal/stt/pair_test.go

@@ -86,6 +86,24 @@ func TestPairFallsBackWhenRemoteFails(t *testing.T) {
 	}
 }
 
+func TestPairFallsBackWhenRemoteReturnsBlankSuccess(t *testing.T) {
+	t.Parallel()
+	remote := &scripted{text: "   "}
+	floor := &scripted{text: "с homesrv"}
+	p := up(remote, floor)
+
+	text, _, err := p.Transcribe(context.Background(), sample())
+	if err != nil {
+		t.Fatalf("invalid remote success must fall back: %v", err)
+	}
+	if text != "с homesrv" || floor.calls.Load() != 1 {
+		t.Fatalf("got text=%q floor calls=%d, want floor transcript once", text, floor.calls.Load())
+	}
+	if p.Available() {
+		t.Fatal("invalid remote response must correct cached availability")
+	}
+}
+
 func TestPairWithNoRemoteIsTheFloor(t *testing.T) {
 	t.Parallel()
 	floor := &scripted{text: "с homesrv"}