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/mux/codec/mpeg2.rs

@@ -195,12 +195,7 @@ fn find_start_code(data: &[u8], from: usize) -> Option<usize> {
     if data.len() < from + 3 {
         return None;
     }
-    for i in from..data.len() - 2 {
-        if data[i] == 0x00 && data[i + 1] == 0x00 && data[i + 2] == 0x01 {
-            return Some(i);
-        }
-    }
-    None
+    (from..data.len() - 2).find(|&i| data[i] == 0x00 && data[i + 1] == 0x00 && data[i + 2] == 0x01)
 }
 
 #[cfg(test)]