Zero clippy warnings: fix all 32 remaining

- Iterator::find() replaces manual loops (6 sites)
- Index-only loops → iterators (4 sites)
- Identical if-blocks merged
- Box large MkvStream WriteState enum variant
- Vec macro initializers, late init fixes
- Unused fields prefixed with underscore (format spec fields)
- Dead code removed or documented

0 clippy warnings. 319 tests passing.

src/aacs/keydb.rs

@@ -57,7 +57,7 @@ pub struct DiscEntry {
 /// Parse a hex string like "0xABCD..." into bytes.
 pub(crate) fn parse_hex(s: &str) -> Option<Vec<u8>> {
     let s = s.trim().trim_start_matches("0x").trim_start_matches("0X");
-    if s.len() % 2 != 0 {
+    if !s.len().is_multiple_of(2) {
         return None;
     }
     let mut out = Vec::with_capacity(s.len() / 2);