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/mux/ebml.rs

@@ -424,6 +424,18 @@ pub const DEFAULT_DURATION: u32 = 0x23_E383;
 pub const VIDEO: u32 = 0xE0;
 pub const PIXEL_WIDTH: u32 = 0xB0;
 pub const PIXEL_HEIGHT: u32 = 0xBA;
+// Scan type (children of Video).
+pub const FLAG_INTERLACED: u32 = 0x9A;
+pub const FIELD_ORDER: u32 = 0x9D;
+// FlagInterlaced values: 1 = interlaced, 2 = progressive (0 = undetermined).
+pub const INTERLACED_INTERLACED: u64 = 1;
+pub const INTERLACED_PROGRESSIVE: u64 = 2;
+// FieldOrder values (Matroska): 0/2 = top-field-first, 1/9 = bottom-field-first.
+// NTSC DVD (480i) and HD (1080i) are top-field-first; PAL DVD (576i) is
+// bottom-field-first. 0xFF is our sentinel for "undetermined / omit".
+pub const FIELD_ORDER_TFF: u8 = 2;
+pub const FIELD_ORDER_BFF: u8 = 9;
+pub const FIELD_ORDER_UNDETERMINED: u8 = 0xFF;
 pub const DISPLAY_WIDTH: u32 = 0x54B0;
 pub const DISPLAY_HEIGHT: u32 = 0x54BA;
 pub const COLOUR: u32 = 0x55B0;