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/labels/pixelogic.rs

@@ -108,9 +108,7 @@ fn parse_token(s: &str) -> Option<StreamLabel> {
         } else if part == "ACOM" {
             purpose = LabelPurpose::Commentary;
             is_audio = true;
-        } else if part == "ADLG" {
-            is_audio = true;
-        } else if part == "ATRI" {
+        } else if part == "ADLG" || part == "ATRI" {
             is_audio = true;
         } else if part == "SDH" {
             qualifier = LabelQualifier::Sdh;
@@ -120,9 +118,7 @@ fn parse_token(s: &str) -> Option<StreamLabel> {
         } else if part == "SCOM" {
             purpose = LabelPurpose::Commentary;
             is_subtitle = true;
-        } else if part == "STRI" {
-            is_subtitle = true;
-        } else if part == "TXT" {
+        } else if part == "STRI" || part == "TXT" {
             is_subtitle = true;
         } else if part == "FOR" {
             qualifier = LabelQualifier::Forced;