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.

src/disc/bluray.rs

@@ -148,6 +148,17 @@ impl Disc {
                         // the CLI/UI render the localized descriptor. `label`
                         // stays empty for disc video streams.
                         label: String::new(),
+                        // TODO(spec): for 1080i HEVC/H.264/VC-1 titles, surface
+                        // the measured field order (H.264/HEVC pic_struct, VC-1
+                        // pulldown) from the codec parser instead of the TFF
+                        // fallback; needs the parser→title channel (see dvd.rs).
+                        top_field_first: None,
+                        // TODO(spec): prefer the HEVC/H.264 VUI colour_description
+                        // (measured CICP) over this MPLS playlist-nibble guess
+                        // once the parser surfaces it through the output title.
+                        // `None` keeps the enum fallback. (HDR MaxCLL/Mastering
+                        // metadata is a separate task and intentionally not here.)
+                        measured_cicp: None,
                     })),
                     2 | 5 => {
                         // Guard: if coding_type is a subtitle codec (PGS 0x90/0x91),