libfreemkv: rc.5.1 DVD correctness fixes

- CSS: unlock scrambled-sector reads on enforcing drives via bus-auth
  only; classify sense 6F/03 as CSS-locked; early-bail on a fully locked
  scan; gate the AACS handshake off DVD discs.
- DVD first-play menu no longer prepended to the feature: read the title
  VOBS base from vtstt_vobs (0xC4), not the menu VOBS vtsm_vobs (0xC0).
- Interlaced field-duration (DefaultDecodedFieldDuration) written as a
  direct TrackEntry child rather than inside Video, so Windows reports
  the correct frame rate.
- Audio channel count read from the AC-3 bitstream; FieldOrder set to
  TFF; per-track BPS tags.
- Structured disc diagnostics at --log-level 3; reduced per-operation
  log spam.

src/mux/ebml.rs

@@ -419,6 +419,11 @@ pub const CODEC_ID: u32 = 0x86;
 pub const CODEC_PRIVATE: u32 = 0x63A2;
 pub const TRACK_NAME: u32 = 0x536E;
 pub const DEFAULT_DURATION: u32 = 0x23_E383;
+/// DefaultDecodedFieldDuration — nanoseconds per FIELD (half a frame for
+/// interlaced content). Emitting it on an interlaced track tells a reader the
+/// field rate so it stops halving the frame rate (Windows shell shows 12.5 fps
+/// for a 25 fps 576i stream without it). RFC 9559 / Matroska v4.
+pub const DEFAULT_DECODED_FIELD_DURATION: u32 = 0x23_4E7A;
 
 // Video
 pub const VIDEO: u32 = 0xE0;
@@ -434,6 +439,10 @@ pub const INTERLACED_PROGRESSIVE: u64 = 2;
 // 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;
+// Bottom-field-first. Retained for completeness/round-trip tests; the muxer
+// emits TFF for all DVD/HD interlaced content (DV is the only common BFF
+// source and freemkv does not produce it).
+#[allow(dead_code)]
 pub const FIELD_ORDER_BFF: u8 = 9;
 pub const FIELD_ORDER_UNDETERMINED: u8 = 0xFF;
 pub const DISPLAY_WIDTH: u32 = 0x54B0;
@@ -472,6 +481,18 @@ pub const CUE_TRACK_POSITIONS: u32 = 0xB7;
 pub const CUE_TRACK: u32 = 0xF7;
 pub const CUE_CLUSTER_POSITION: u32 = 0xF1;
 
+// Tags — per-track statistics tags. mkvmerge convention: a `BPS` SimpleTag
+// per track carries the bits-per-second so readers (Windows Explorer's MKV
+// property handler) that read the container tag rather than computing from
+// stream size show a bitrate for every track, not just CBR audio.
+pub const TAGS: u32 = 0x1254_C367;
+pub const TAG: u32 = 0x7373;
+pub const TARGETS: u32 = 0x63C0;
+pub const TAG_TRACK_UID: u32 = 0x63C5;
+pub const SIMPLE_TAG: u32 = 0x67C8;
+pub const TAG_NAME: u32 = 0x45A3;
+pub const TAG_STRING: u32 = 0x4487;
+
 // Chapters
 pub const CHAPTERS: u32 = 0x1043_A770;
 pub const EDITION_ENTRY: u32 = 0x45B9;