DVD: correct PAL/NTSC, anamorphic aspect, and SD colour

Fix three DVD video-attribute bugs surfaced by a PAL disc detected as
NTSC:

- PAL/NTSC: parse video_format from VTS_V_ATR bits 5-4, not bits 1-0
  (the old mask read permitted_df, so PAL 576i/25fps was mis-detected
  as NTSC 480i/29.97). Named consts replace the magic bit positions.
- Anamorphic aspect: write MKV DisplayWidth/Height from the disc's
  display_aspect (16:9 720x576 -> 1024x576) instead of square pixels,
  so 16:9 DVDs no longer render as 4:3.
- Colour: stamp SD colorimetry (PAL=BT.470BG, NTSC=SMPTE-170M) instead
  of BT.709 (HD).

Adds VideoStream.display_aspect (threaded through every muxer) plus
TvSystem/DvdAspect/ColorSpace plumbing, with regression tests. Removes
the deprecated Disc mux set_halt bridge (use with_halt).

src/mux/mkvstream.rs

@@ -582,6 +582,9 @@ fn parse_track(
                 frame_rate: FrameRate::Unknown,
                 hdr: HdrFormat::Sdr,
                 color_space: ColorSpace::Bt709,
+                // Remux input: the source MKV's DisplayWidth/Height is preserved
+                // by the writer separately; nothing anamorphic to reconstruct here.
+                display_aspect: None,
                 secondary: is_secondary,
                 label: name,
             }))