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/truehd.rs

@@ -126,19 +126,22 @@ enum Ac3Size {
 
 impl CodecParser for TrueHdParser {
     fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
-        if pes.data.is_empty() {
-            return Vec::new();
-        }
-
         // B1: a concealed/lost gap means the buffered TrueHD AU is TRUNCATED.
         // Splicing post-gap bytes onto it corrupts the AU framing (→ "Invalid
         // data found") and strands the PTS cadence (the non-monotonic audio-DTS
         // band at gaps). Drop the partial; with `buf` now empty the PTS-base block
         // below re-seeds the cadence from the post-gap PES, monotonic across the
         // gap. (Audio has no inter-frame refs — this is the whole audio fix.)
+        //
+        // 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();
         }
+        if pes.data.is_empty() {
+            return Vec::new();
+        }
 
         // Capture the PTS base ONLY at an access-unit boundary, i.e. when no AU
         // is mid-assembly in `buf`. TrueHD access units span PES packets; a PES