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

.github/workflows/ci.yml

@@ -32,6 +32,7 @@ jobs:
             dart:
               - 'lib/**'
               - 'test/**'
+              - 'android/app/src/test/resources/**'
               - 'assets/**'
               - 'pubspec.yaml'
               - 'pubspec.lock'
@@ -41,6 +42,7 @@ jobs:
               - '.github/workflows/ci.yml'
             go:
               - 'go_backend/**'
+              - 'android/app/src/test/resources/**'
               - '.github/workflows/ci.yml'
             android:
               - 'android/**'

CONTRIBUTING.md

@@ -112,6 +112,10 @@ Dart should consume that declaration without knowing which extension uses it.
 Run checks that cover the code you changed. Before opening a PR, the relevant
 commands should pass.
 
+Cross-language lyric usability cases live in
+`android/app/src/test/resources/lyrics_usability_cases.tsv`. Dart, Go, and
+Android tests read the same cases; add a case there when changing that policy.
+
 Flutter and Dart:
 
 ```bash

android/app/src/test/kotlin/com/zarz/spotiflac/NativeFinalizationPolicyTest.kt

@@ -7,6 +7,29 @@ import org.junit.Assert.assertTrue
 import org.junit.Test
 
 class NativeFinalizationPolicyTest {
+    @Test
+    fun matchesSharedLyricUsabilityCases() {
+        val stream = checkNotNull(
+            javaClass.getResourceAsStream("/lyrics_usability_cases.tsv"),
+        )
+        stream.bufferedReader().useLines { lines ->
+            for (line in lines) {
+                if (line.isBlank() || line.startsWith("#")) continue
+                val fields = line.split('\t')
+                assertEquals("invalid shared fixture: $line", 3, fields.size)
+                val lyrics = fields[2]
+                    .replace("\\n", "\n")
+                    .replace("\\r", "\r")
+                    .replace("\\t", "\t")
+                assertEquals(
+                    fields[0],
+                    fields[1].toBooleanStrict(),
+                    NativeFinalizationPolicy.hasUsableLyricsContent(lyrics),
+                )
+            }
+        }
+    }
+
     @Test
     fun usableLyricsRejectsHeadersButKeepsRealAndInstrumentalContent() {
         assertFalse(

android/app/src/test/resources/lyrics_usability_cases.tsv

@@ -0,0 +1,23 @@
+# Shared Dart/Go/Kotlin lyric-usability cases.
+# Columns: name<TAB>expected boolean<TAB>lyrics. Decode literal \n, \r, \t in lyrics.
+empty	false	
+whitespace	false	 \t\r\n 
+metadata_only	false	[ar:Artist]\n[ti:Title]\n[offset:0]
+metadata_case	false	[AR:Artist]\n[BY:SpotiFLAC]
+instrumental	true	 [Instrumental:TRUE] 
+timestamps_only	false	[00:01.00]\n<00:01.10>
+multiple_timestamps	false	[00:01.00][01:02:500] 
+speaker_only	false	v1:\n V2: 
+inline_before_spaced_speaker	false	[00:00.00]<00:00.01> v1:
+inline_before_tabbed_speaker	false	[00:00.00]<00:00.01>\tV2:\t
+multiple_inline_speaker	false	[00:00.00]<00:00.01> <00:00.02> v2: 
+empty_background	false	[bg: ]
+empty_timed_background	false	[bg:[00:00.00]<00:00.01> v1: ]
+plain_text	true	First line\nSecond line
+timed_text	true	[00:01.00]First line
+timed_speaker_text	true	[00:02:500]<00:02.500> v2: Harmony line
+background_text	true	[bg:Backing vocal]
+timed_background_text	true	[BG:[00:00.00]<00:00.01> v1: Backing vocal]
+multiple_timed_text	true	[00:01.00][00:02.00]Repeated line
+metadata_and_text	true	[ti:Title]\n[00:00.00]Actual lyric
+unicode_text	true	[00:00.00]<00:00.01> v1: 日本語の歌詞

go_backend/lyrics_usability_test.go

@@ -0,0 +1,35 @@
+package gobackend
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestSharedLyricsUsabilityCases(t *testing.T) {
+	data, err := os.ReadFile(filepath.Join("..", "android", "app", "src", "test", "resources", "lyrics_usability_cases.tsv"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	decode := strings.NewReplacer(`\n`, "\n", `\r`, "\r", `\t`, "\t")
+	for _, line := range strings.Split(string(data), "\n") {
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		fields := strings.Split(line, "\t")
+		if len(fields) != 3 {
+			t.Fatalf("invalid shared fixture: %q", line)
+		}
+		t.Run(fields[0], func(t *testing.T) {
+			want, err := strconv.ParseBool(fields[1])
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got := rawLyricsHasUsableContent(decode.Replace(fields[2])); got != want {
+				t.Errorf("rawLyricsHasUsableContent(%q) = %v, want %v", fields[2], got, want)
+			}
+		})
+	}
+}

lib/utils/lyrics_metadata_helper.dart

@@ -48,7 +48,7 @@ String cleanLyricsForDisplay(String lyrics) {
     while (_lrcDisplayTimestampPattern.hasMatch(cleaned)) {
       cleaned = cleaned.replaceFirst(_lrcDisplayTimestampPattern, '').trim();
     }
-    cleaned = cleaned.replaceAll(_lrcDisplayInlineTimestampPattern, '');
+    cleaned = cleaned.replaceAll(_lrcDisplayInlineTimestampPattern, '').trim();
     cleaned = cleaned.replaceFirst(_lrcDisplaySpeakerPrefixPattern, '');
     cleaned = cleaned.replaceAll(RegExp(r'\s+'), ' ').trim();
 

test/lyrics_metadata_helper_test.dart

@@ -1,7 +1,27 @@
+import 'dart:io';
+
 import 'package:flutter_test/flutter_test.dart';
 import 'package:spotiflac_android/utils/lyrics_metadata_helper.dart';
 
 void main() {
+  group('shared lyric usability cases', () {
+    final cases = File(
+      'android/app/src/test/resources/lyrics_usability_cases.tsv',
+    ).readAsLinesSync();
+    for (final line in cases) {
+      if (line.isEmpty || line.startsWith('#')) continue;
+      final fields = line.split('\t');
+      test(fields.first, () {
+        expect(fields, hasLength(3));
+        final lyrics = fields[2]
+            .replaceAll(r'\n', '\n')
+            .replaceAll(r'\r', '\r')
+            .replaceAll(r'\t', '\t');
+        expect(hasUsableLyricsContent(lyrics), fields[1] == 'true');
+      });
+    }
+  });
+
   group('lyrics display normalization', () {
     test('rejects metadata-only embedded LRC', () {
       const raw = '''