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/handshake.rs

@@ -1137,9 +1137,9 @@ mod tests {
     #[test]
     fn test_ecdh_shared_secret() {
         // Two parties should derive the same shared point
-        let p = BigUint::from_bytes_be(&EC_P);
-        let a = BigUint::from_bytes_be(&EC_A);
-        let g = EcPoint::from_bytes(&EC_GX, &EC_GY);
+        let _p = BigUint::from_bytes_be(&EC_P);
+        let _a = BigUint::from_bytes_be(&EC_A);
+        let _g = EcPoint::from_bytes(&EC_GX, &EC_GY);
 
         let (priv_a, pub_ax, pub_ay) = generate_host_key_pair();
         let (priv_b, pub_bx, pub_by) = generate_host_key_pair();

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);