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/meta.rs

@@ -209,6 +209,7 @@ impl M2tsMeta {
                             .unwrap_or(crate::disc::FrameRate::Unknown),
                         hdr: hdr_fmt,
                         color_space: cs,
+                        display_aspect: None,
                         secondary: *secondary,
                         label: label.clone(),
                     })
@@ -403,6 +404,7 @@ mod tests {
             frame_rate: FrameRate::F23_976,
             hdr,
             color_space: cs,
+            display_aspect: None,
             secondary: false,
             label: String::new(),
         }));
@@ -651,6 +653,7 @@ mod tests {
                     frame_rate: FrameRate::F23_976,
                     hdr: HdrFormat::Hdr10,
                     color_space: ColorSpace::Bt2020,
+                    display_aspect: None,
                     secondary: false,
                     label: "x".into(),
                 }));