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/disc/bluray.rs

@@ -137,6 +137,10 @@ impl Disc {
                             2 => ColorSpace::Bt2020,
                             _ => ColorSpace::Unknown,
                         },
+                        // Blu-ray HD/UHD video is square-pixel; display aspect
+                        // equals the pixel grid (16:9). Anamorphic SD-on-BD is
+                        // not special-cased here.
+                        display_aspect: None,
                         secondary: s.secondary,
                         // No user-facing English in the library (numeric-code
                         // rule): the Dolby Vision enhancement layer is signalled