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/ts.rs

@@ -690,6 +690,8 @@ pub fn scan_streams(data: &[u8]) -> Option<Vec<crate::disc::Stream>> {
                         frame_rate: FrameRate::Unknown,
                         hdr: HdrFormat::Sdr,
                         color_space: ColorSpace::Bt709,
+                        // TS is a passthrough container — aspect stays in the ES.
+                        display_aspect: None,
                         secondary: false,
                         label: String::new(),
                     }))