Preserve interlaced scan type in label and MKV output

Interlacing is detected upstream (PAL DVD -> R576i) but was dropped in
two places: the video label hardcoded a 'p' suffix, and the muxer never
wrote any scan-type flag, so MediaInfo inferred progressive and reported
576p for a 576i source.

- Add Resolution::is_interlaced() for the R*i variants.
- generate_video_label now branches i/p for the heights that can be
  interlaced (1080, 576, 480) instead of always emitting 'p'.
- MkvTrack carries interlaced + field_order; the video serializer emits
  FlagInterlaced (0x9A; 1=interlaced, 2=progressive) and, for interlaced
  content, FieldOrder (0x9D) - bottom-field-first for PAL 576i,
  top-field-first otherwise. Adds the EBML constants.

src/disc/mod.rs

@@ -655,6 +655,14 @@ impl Resolution {
         matches!(self, Resolution::R2160p | Resolution::R4320p)
     }
 
+    /// True if this is an interlaced resolution (the `R*i` variants).
+    pub fn is_interlaced(&self) -> bool {
+        matches!(
+            self,
+            Resolution::R480i | Resolution::R576i | Resolution::R1080i
+        )
+    }
+
     /// True if this is an HD (720p+) resolution.
     pub fn is_hd(&self) -> bool {
         !matches!(