Lint the test code, and fix the 74 findings it had been hiding

Every other repo's CI now runs clippy with --all-targets. libfreemkv,
the crate the other seven build against and the one held up as the
reference workflow, was the last one still linting the library only — so
its ~3,000 tests, by far the largest body of test code in the project,
had never been linted at all. Turning the flag on surfaced 74 findings.

Most were mechanical and applied with clippy --fix. The rest, by hand:

- Four discarded Results in decrypt.rs. css::descramble_region returns a
  Result and four CSS tests threw it away, so a descramble that FAILED
  would have surfaced as a confusing buffer-comparison mismatch instead
  of the actual error. They expect() now.
- A dead `kp` field on the PlantedWalk fixture. The test deliberately
  asserts Kp as the explicit AES-G3(dk, 1) relation from [C] §3.2.4
  rather than against a stored value — its doc comment says so — which
  makes the field not just unused but a trap: the obvious "fix" of
  asserting against it would quietly weaken the test to comparing the
  fixture with itself. Removed.
- Two hand-rolled ICB counters in the HD-DVD fixtures, a needless mut,
  three vec!s that only ever needed arrays, a filter_map whose every arm
  was Some, and a Vec::new()+push chain.
- Doc list indentation in mkv.rs and mp4/read.rs, which was mis-rendering
  in the generated docs.
- A five-[u8; 16]-tuple return type named FourLevelParts.

Three lints are allowed at the specific sites, with reasons, because
they are wrong for this domain: the underscores in the bitstream-header
literals mark BITFIELD boundaries, not digit groups, so regrouping them
uniformly would satisfy the lint by destroying the only thing they
encode; and in three table-validation loops the loop variable is the
domain value under test (a DTS SFREQ code, an AMODE value, a palette
entry number), which is what the assertion messages name.

src/mux/mp4/read.rs

@@ -1087,6 +1087,11 @@ mod tests {
     }
 
     #[test]
+    // The underscores in these literals mark BITFIELD boundaries in the
+    // bitstream header being built (e.g. a 5-bit field then a 3-bit field),
+    // not thousands-style digit groups. Regrouping them uniformly would
+    // satisfy the lint by destroying the only thing they encode.
+    #[allow(clippy::unusual_byte_groupings)]
     fn write_then_read_round_trip() {
         // Mux a small A/V title to an in-memory MP4, then demux it back and
         // check the streams, codec_private, and sample payloads survive.
@@ -1453,6 +1458,11 @@ mod tests {
     /// one of any of them round-trips through this crate unnoticed while making
     /// the file unplayable elsewhere.
     #[test]
+    // The underscores in these literals mark BITFIELD boundaries in the
+    // bitstream header being built (e.g. a 5-bit field then a 3-bit field),
+    // not thousands-style digit groups. Regrouping them uniformly would
+    // satisfy the lint by destroying the only thing they encode.
+    #[allow(clippy::unusual_byte_groupings)]
     fn moov_tree_carries_the_mandatory_track_header_and_media_boxes() {
         use crate::disc::{
             AudioChannels, AudioStream, Codec, DiscTitle, FrameRate, HdrFormat, LabelPurpose,
@@ -2148,8 +2158,8 @@ mod tests {
         mdia.extend_from_slice(&mdhd);
         mdia.extend_from_slice(&hdlr);
         mdia.extend_from_slice(&minf);
-        let trak = mp4_box(b"trak", &mp4_box(b"mdia", &mdia));
-        trak
+
+        mp4_box(b"trak", &mp4_box(b"mdia", &mdia))
     }
 
     #[test]
@@ -2449,10 +2459,10 @@ mod tests {
     }
 
     /// A `VisualSampleEntry` body with DISTINCT width and height, plus optional
-    /// child boxes (ISO/IEC 14496-12 §12.1.3): 6 reserved + 2 data_reference_index
-    /// + 16 pre_defined/reserved, then width(2) at 24 and height(2) at 26, then 50
-    /// more bytes of resolution / frame_count / compressorname / depth / pre_defined
-    /// to the 78-byte fixed part.
+    /// child boxes (ISO/IEC 14496-12 §12.1.3). The fixed part is 78 bytes:
+    /// 6 reserved, 2 data_reference_index, 16 pre_defined/reserved, width(2) at
+    /// offset 24, height(2) at 26, then 50 more bytes of resolution,
+    /// frame_count, compressorname, depth and pre_defined.
     fn visual_entry(width: u16, height: u16, children: &[u8]) -> Vec<u8> {
         let mut b = vec![0u8; 78];
         b[24..26].copy_from_slice(&width.to_be_bytes());
@@ -2492,7 +2502,7 @@ mod tests {
 
         // And a VisualSampleEntry too short to hold the fixed part is refused
         // rather than read out of a shorter buffer.
-        let short = stsd_with(b"avc1", &vec![0u8; 40]);
+        let short = stsd_with(b"avc1", &[0u8; 40]);
         assert!(
             parse_stsd(&short).is_none(),
             "a truncated VisualSampleEntry has no dimensions to read"
@@ -2519,7 +2529,7 @@ mod tests {
         assert_eq!(info.height, 0, "an audio entry declares no height");
 
         // Too short for the 28-byte fixed part: fall back to stereo, not to 0.
-        let short = stsd_with(b"ac-3", &vec![0u8; 12]);
+        let short = stsd_with(b"ac-3", &[0u8; 12]);
         let info = parse_stsd(&short).expect("a short audio entry still names a codec");
         assert_eq!(
             info.channels, 2,