mux: HD-DVD VC-1 demux via extended stream id 0xFD

VC-1 HD-DVDs (e.g. Shaun of the Dead) carry video on MPEG-PS extended
stream id 0xFD, with the real stream selector in stream_id_extension
inside the PES extension. Parse that field so the video routes to a
distinct track (pid 0xFD00|ext) instead of being dropped.

Reframe VC-1 access units in AuAssembler with a dedicated Mode::Vc1:
an AU is delimited by the next frame BDU (0x0D) once a frame has already
been seen, so the sequence (0x0F) and entry-point (0x0E) headers that
precede an I-frame stay attached to the frame they describe. The old
single-start-code split stranded those headers on the prior AU, which
the decoder reported as bits-overconsumption and hard decode failures.

hddvd probe now tracks the video pid it detects and emits VC-1 on 0xFD.

src/mux/mod.rs

@@ -39,6 +39,7 @@ pub mod resolve;
 // accessors and an alternate `DemuxThread` spawn path. They are kept as
 // part of the parser/demux surface and covered by unit tests; allow the
 // dead-code lint rather than delete still-relevant scaffolding.
+pub(crate) mod au_assembly;
 #[allow(dead_code)]
 pub(crate) mod codec;
 pub(crate) mod demux_sink;