Test the seam gate that stops a drift-broken title being skipped

Audit finding: both seam gates in MkvMuxer::finish were dead code under test.
set_clips was never called anywhere in the suite, so continuity.dropped_total()
was always zero and neither branch could be reached.

The ordering of the two zero-frame checks is load-bearing and the comment says
so: a title the seam plan emptied also has a zero frame count, and MkvInvalid
is classified by is_skippable_title_stub as an empty nav/menu stub — so
reporting it that way makes an all-titles rip drop a real feature and finish the
rest at exit 0. Swapping the checks left the entire suite green.

The new test builds a two-clip plan (from_clips needs at least two, strictly
increasing) whose marks exclude every frame, and asserts both that the error is
SinkWroteNothing and that it is NOT classified as skippable. Confirmed to FAIL
with the two checks swapped.

src/mux/mkv.rs

@@ -4452,6 +4452,58 @@ mod tests {
         assert_eq!(err.kind(), io::ErrorKind::InvalidData);
     }
 
+    /// A seam-plan clip list whose marks exclude every frame must fail with
+    /// `SinkWroteNothing`, NOT `MkvInvalid`.
+    ///
+    /// Audit finding: both gates in `finish()` were dead code under test —
+    /// `set_clips` was never called anywhere in the suite, so
+    /// `continuity.dropped_total()` was always 0 and neither branch could be
+    /// reached. Swapping the two `frame_count == 0` checks left the whole suite
+    /// green, while a title the seam plan emptied came back as `MkvInvalid` —
+    /// which `is_skippable_title_stub` classifies as an empty nav/menu stub, so
+    /// an all-titles rip would silently omit a real feature and exit 0.
+    #[test]
+    fn a_title_the_seam_plan_emptied_is_not_reported_as_a_skippable_stub() {
+        let tracks = [make_video_track()];
+        // TWO clips (SeamPlan::from_clips requires at least two, with strictly
+        // increasing IN marks), covering 100s..200s and 200s..300s in 45 kHz
+        // ticks. Every frame below sits far before the first IN, so the plan
+        // places none of them and drops them all.
+        let clips = [
+            crate::disc::Clip {
+                clip_id: "00000".into(),
+                in_time: 100 * 45_000,
+                out_time: 200 * 45_000,
+                duration_secs: 100.0,
+                source_packets: 0,
+            },
+            crate::disc::Clip {
+                clip_id: "00001".into(),
+                in_time: 200 * 45_000,
+                out_time: 300 * 45_000,
+                duration_secs: 100.0,
+                source_packets: 0,
+            },
+        ];
+        let mut muxer = MkvMuxer::new(Cursor::new(Vec::new()), &tracks, None, 100.0, &[]).unwrap();
+        muxer.set_clips(&clips, crate::disc::ContentFormat::BdTs);
+        // 0s and 1s — both before the clip's IN mark.
+        let _ = muxer.write_frame(0, 0, true, &[0x01; 16], None, None);
+        let _ = muxer.write_frame(0, 1_000_000_000, true, &[0x02; 16], None, None);
+
+        let err = muxer.finish().unwrap_err();
+        assert_eq!(
+            crate::error::error_code(&err),
+            Some(crate::error::E_SINK_WROTE_NOTHING),
+            "an emptied seam title must report SinkWroteNothing"
+        );
+        assert!(
+            !crate::error::is_skippable_title_stub(&err),
+            "it must NOT be classified as a skippable stub — that is what makes an \
+             all-titles rip drop a real feature and still exit 0"
+        );
+    }
+
     #[test]
     fn backjumped_audio_handled_by_i16_split_no_wrap() {
         // A NON-VIDEO (audio) frame whose PTS back-jumps far below the open