mux: harden audio discontinuity handling (audit follow-up)

Two defensive hardenings from the post-fix audit (vs FFmpeg/GStreamer):

1. Move the `pes.discontinuity` partial-drop ABOVE the empty-data guard in
   all three audio parsers (ac3/dts/truehd), so a discontinuity signal can
   never be stranded by an empty post-gap PES. The demuxer only emits
   non-empty PES today; this is defense-in-depth for any future caller.

2. A PES with no PTS must not reset the timeline to 0. ac3 now carries
   `flush_pts_ns`, dts continues from the most recent known base; truehd
   already kept its running cadence on a None PTS. Matches OSS behavior
   (PTS rebases off the next PES that actually carries a PTS).

Adds an ac3 regression test (empty-payload discontinuity PES still drops
the stranded partial). Loss accounting was reviewed: TS-demux CC-gaps are
NOT counted toward lost_video_secs / abort (that is sector-based via
DiscStream::errors / mapfile bytes_unreadable), so a source splice never
inflates loss — no gating needed there.

src/mux/codec/dts.rs

@@ -123,21 +123,39 @@ const PTS_UNSET: i64 = -1;
 
 impl CodecParser for DtsParser {
     fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
-        if pes.data.is_empty() {
-            return Vec::new();
-        }
         // B1: a concealed/lost gap means the buffered DTS access unit is
         // TRUNCATED. Splicing post-gap bytes onto it corrupts the core/extension
         // framing (→ "Failed to decode block code(s)" / "Invalid data found").
         // Drop the partial AU and its PTS marks; the next PES re-bases a fresh
         // unit. (Audio has no inter-frame refs — dropping the spliced partial is
         // the whole fix; the video ResyncGate handles video.)
+        //
+        // Handle the discontinuity BEFORE the empty-data guard so the signal can
+        // never be stranded by an empty post-gap PES (defensive; the demuxer only
+        // emits non-empty PES today).
         if pes.discontinuity {
             self.buf.clear();
             self.pts_marks.clear();
             self.pending_pts = PTS_UNSET;
         }
-        let pts_ns = pes.pts.map(pts_to_ns).unwrap_or(0);
+        if pes.data.is_empty() {
+            return Vec::new();
+        }
+        // A PES with no PTS (rare for audio, but legal — the case OSS demuxers
+        // guard at a post-gap continuation) must NOT reset the timeline to 0;
+        // continue from the most recent known base. Defense-in-depth: the
+        // discontinuity-carrying PES is a PUSI with a PTS in practice.
+        let pts_ns = pes.pts.map(pts_to_ns).unwrap_or_else(|| {
+            self.pts_marks
+                .last()
+                .map(|&(_, p)| p)
+                .filter(|&p| p >= 0)
+                .unwrap_or(if self.pending_pts >= 0 {
+                    self.pending_pts
+                } else {
+                    0
+                })
+        });
 
         // On Blu-ray, a DTS-HD MA/HRA access unit is a DTS core frame
         // (sync 0x7FFE8001) followed by one or more DTS extension substreams