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).

tests/streams.rs

@@ -20,6 +20,7 @@ fn sample_disc_title() -> DiscTitle {
                 frame_rate: FrameRate::F23_976,
                 hdr: HdrFormat::Hdr10,
                 color_space: ColorSpace::Bt709,
+                display_aspect: None,
                 secondary: false,
                 label: "Main".into(),
             }),
@@ -397,6 +398,7 @@ fn meta_codec_roundtrip() {
             frame_rate: FrameRate::F23_976,
             hdr: HdrFormat::Sdr,
             color_space: ColorSpace::Bt709,
+            display_aspect: None,
             secondary: false,
             label: String::new(),
         }));
@@ -498,6 +500,7 @@ fn meta_all_stream_types() {
                 frame_rate: FrameRate::F23_976,
                 hdr: HdrFormat::Hdr10,
                 color_space: ColorSpace::Bt709,
+                display_aspect: None,
                 secondary: false,
                 label: "Primary".into(),
             }),
@@ -682,6 +685,7 @@ fn mkvstream_meta_preserves_all_streams() {
                 frame_rate: FrameRate::F23_976,
                 hdr: HdrFormat::Sdr,
                 color_space: ColorSpace::Bt709,
+                display_aspect: None,
                 secondary: false,
                 label: "Main Video".into(),
             }),
@@ -780,6 +784,7 @@ fn mkvstream_e2e_h264_produces_valid_mkv() {
             frame_rate: FrameRate::F23_976,
             hdr: HdrFormat::Sdr,
             color_space: ColorSpace::Bt709,
+            display_aspect: None,
             secondary: false,
             label: "Main".into(),
         })],