fix(lyrics): align usability checks with shared fixtures

Trim after removing inline timestamps before matching speaker prefixes in Dart. Run one 21-case usability corpus through Dart, Go, and Kotlin tests.

Document the shared fixture and trigger Dart and Go CI when its resources change.

.gitattributes

@@ -9,6 +9,9 @@ gradlew text eol=lf
 *.cmd text eol=crlf
 *.ps1 text eol=crlf
 
+# Empty TSV fields and trailing lyric whitespace are intentional test data.
+android/app/src/test/resources/lyrics_usability_cases.tsv whitespace=-blank-at-eol
+
 # Binary files
 *.png binary
 *.jpg binary