Test the demux seam gates too

The same gap as the MKV muxer: DemuxSink builds its timeline with
TimelineContinuity::with_clips, but no test ever handed it a title carrying
usable PlayItem marks, so frames_mapped and dropped_total() were always zero
and both gates in finish() were unreachable.

Deleting either left the whole suite green while a demux:// export of a
seamless-branching title wrote a directory of zero-byte track files beside a
populated chapters document and reported success at exit 0.

The test gives the title two clips with marks that exclude every frame, and was
confirmed to FAIL with the gate removed.

src/mux/demux_sink.rs

@@ -1680,6 +1680,63 @@ mod tests {
         let _ = std::fs::remove_dir_all(&dir);
     }
 
+    /// A demux export whose seam plan drops every frame must FAIL, not write a
+    /// directory of empty track files and report success.
+    ///
+    /// Audit finding: this sink builds its timeline with
+    /// `TimelineContinuity::with_clips` but no test ever gave it a title with
+    /// usable PlayItem marks, so `frames_mapped` and `dropped_total()` were
+    /// always zero and BOTH gates in `finish` were unreachable. Deleting either
+    /// left the whole suite green while a `demux://` export of a
+    /// seamless-branching title wrote zero-byte track files beside a populated
+    /// chapters document, at exit 0.
+    #[test]
+    fn a_demux_export_the_seam_plan_emptied_fails() {
+        let dir = tempdir();
+        let mut title = title_with(vec![video_stream(Codec::H264)], vec![None]);
+        // Two clips (SeamPlan needs >= 2 with strictly increasing IN marks)
+        // covering 100s..200s and 200s..300s in 45 kHz ticks.
+        title.clips = vec![
+            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 sink = DemuxSink::create(&dir, &title, &DemuxOptions::default()).unwrap();
+
+        // Frames far before the first IN mark: the plan places none of them.
+        for (i, pts) in [0i64, 1_000_000_000].iter().enumerate() {
+            let f = PesFrame {
+                coding: None,
+                source: None,
+                track: 0,
+                pts: *pts,
+                keyframe: true,
+                data: vec![0x00, 0x00, 0x00, 0x01, 0x09, 0x10, i as u8],
+                duration_ns: None,
+            };
+            let _ = Stream::write(&mut sink, &f);
+        }
+
+        let err = Stream::finish(&mut sink).expect_err("an emptied export must fail");
+        assert_eq!(
+            crate::error::error_code(&err),
+            Some(crate::error::E_SINK_WROTE_NOTHING),
+            "a fully-dropped demux export must report SinkWroteNothing, not success"
+        );
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     #[test]
     fn sink_read_returns_write_only() {
         let dir = tempdir();