Chapters, DVD subtitle palette, MKV track flags, progress total_bytes

Chapters:
- MPLS PlayList marks parsed (mark_type 1 = chapter)
- Chapter struct on DiscTitle (time_secs, name)
- MKV Chapters element with EditionEntry/ChapterAtom per mark
- 3 MPLS mark tests + 2 MKV chapter tests

DVD subtitle palette:
- IFO palette extraction (PGC offset 0xA4, 16 × YCbCr colors)
- YCbCr→RGB conversion for VobSub .idx format
- DvdSubParser codec_private returns formatted palette
- codec_data field on SubtitleStream flows through pipeline
- 5 palette tests (YCbCr conversion, formatting, overflow)

MKV track flags:
- FlagDefault: primary video/audio = 1, secondary = 0
- FlagForced: forced subtitles = 1
- Language: set from stream language code
- Already implemented, verified with 4 new tests

Progress total_bytes:
- IOStream trait: total_bytes() -> Option<u64>
- DiscStream, IsoStream: from disc_title.size_bytes
- M2tsStream, MkvStream: from file metadata on open
- NetworkStream, StdioStream, NullStream: None

316 tests total, all passing.

src/mux/ebml.rs

@@ -408,6 +408,16 @@ pub const CUE_TRACK_POSITIONS: u32 = 0xB7;
 pub const CUE_TRACK: u32 = 0xF7;
 pub const CUE_CLUSTER_POSITION: u32 = 0xF1;
 
+// Chapters
+pub const CHAPTERS: u32 = 0x1043A770;
+pub const EDITION_ENTRY: u32 = 0x45B9;
+pub const CHAPTER_ATOM: u32 = 0xB6;
+pub const CHAPTER_UID: u32 = 0x73C4;
+pub const CHAPTER_TIME_START: u32 = 0x91;
+pub const CHAPTER_DISPLAY: u32 = 0x80;
+pub const CHAP_STRING: u32 = 0x85;
+pub const CHAP_LANGUAGE: u32 = 0x437C;
+
 // Track types
 pub const TRACK_TYPE_VIDEO: u64 = 1;
 pub const TRACK_TYPE_AUDIO: u64 = 2;