Fix DTS-HD MA muxing as lossy core only (#10)

DTS-HD MA/HRA access units on Blu-ray are a DTS core frame (sync
0x7FFE8001) followed by one or more DTS extension substreams (sync
0x64582025) carrying the lossless audio. Ground-truthing the Dunkirk
ISO showed the m2ts demuxer hands these out as SEPARATE PES packets on
the same PID: one core PES (exactly core-sized, nothing trailing), then
the extension substreams in following PES packets with their own later
PTS.

The old DtsParser emitted one frame per PES the moment a core frame was
complete, and dropped any PES with no core sync. So every core became a
core-only (lossy) frame and the extension PES packets were discarded as
junk -- silently downgrading the track to lossy DTS core (1557 kb/s CBR,
16-bit) instead of DTS-HD MA (VBR, 24-bit lossless).

Rewrite the parser to assemble across PES boundaries: an access unit
runs from its core sync up to (but not including) the NEXT core sync, so
the core plus every following extension substream stays together. Add a
CodecParser::flush() (default empty) called at end-of-stream by both the
pipelined and inline DiscStream mux paths to drain the final buffered
unit. A 64 KiB cap guarantees forward progress and never stalls if a
boundary can't be found.

Validated on the rip1 testbed: Dunkirk eng+ger and Fight Club eng main
audio now ffprobe as profile=DTS-HD MA (Fight Club eng at 24-bit), with
VBR packet sizes (~2716-2788 B) well above the old fixed 2012 B lossy
core. Genuinely-lossy DTS dub tracks are left untouched.

src/mux/codec/mod.rs

@@ -52,6 +52,18 @@ pub trait CodecParser: Send {
     /// Some (TrueHD): multiple access units per PES.
     fn parse(&mut self, pes: &PesPacket) -> Vec<Frame>;
 
+    /// Drain any access unit still buffered after the last PES.
+    ///
+    /// Parsers that buffer across PES boundaries to assemble a complete
+    /// access unit (e.g. DTS-HD, whose extension substreams arrive in
+    /// separate PES packets) hold the final unit until they can prove it's
+    /// complete. At end-of-stream there is no following packet to prove it,
+    /// so the demuxer calls `flush()` once after the last PES to emit it.
+    /// Default: nothing buffered, no tail.
+    fn flush(&mut self) -> Vec<Frame> {
+        Vec::new()
+    }
+
     /// Get codec initialization data (e.g., SPS+PPS for H.264).
     /// Returns None until enough data has been seen.
     fn codec_private(&self) -> Option<Vec<u8>>;