mux/mkv: spec-conformance fixes (field order, CICP, VobSub idx, SeekHead, AC-3)

- FieldOrder now derives from the bitstream's measured top_field_first
  (Some(true)→TFF, Some(false)→BFF) instead of hardcoding TFF for all
  interlaced content; falls back to TFF when unmeasured. Adds
  VideoStream::top_field_first; DVD/BD scan sets None with a precise
  TODO(spec) for parser→title plumbing.
- CICP (matrix/transfer/primaries/range) now prefers measured CICP from
  the bitstream (VideoStream::measured_cicp) over the coarse ColorSpace
  enum, so the container stops assuming a colour space the stream may
  contradict. Enum remains the fallback.
- VobSub S_VOBSUB CodecPrivate now emits a `size: WxH` line ahead of the
  palette per the .idx format so players place/scale subs correctly.
- SeekHead: when zero cues are written, the CUES Seek entry is Voided
  instead of leaving a dangling pointer to the Cues offset (now Tags/EOF).
- AC-3 Channels back-patch offset is captured from the writer instead of
  the hardcoded chan_elem_pos+2 (decoupled from the VINT width choice).
- Hoisted inline CICP codes and the dvcC fourcc to named constants citing
  ITU-T H.273 / RFC 9559; fixed the stale FieldOrder comment.
- DefaultDuration vs pulldown: precise TODO(spec) left (needs the same
  parser→title channel as top_field_first).

Tests: BFF-from-measured-flag, measured-CICP-overrides-enum, VobSub size:
line present/omitted, zero-cue SeekHead Void. precommit (1.86) green.

tests/disc_tests.rs

@@ -169,6 +169,8 @@ fn title_with_video(
             display_aspect: None,
             secondary: false,
             label: String::new(),
+            top_field_first: None,
+            measured_cicp: None,
         })],
         chapters: Vec::new(),
         extents: Vec::new(),