fix(playback): retain ReplayGain for SAF descriptor leases

Carry the original display name through playback leases to format-aware metadata reads. Cache successful normalization results by source identity, bound the cache, and retry transient read failures.

Add regressions for descriptor hints, album gain fallback, and failed-read retries.

test/playback_normalization_test.dart

@@ -0,0 +1,81 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/playback_normalization.dart';
+
+void main() {
+  test(
+    'descriptor normalization uses hint and caches by original URI',
+    () async {
+      var reads = 0;
+      final cache = PlaybackNormalizationCache(
+        readMetadata: (path, {displayName}) async {
+          reads++;
+          expect(path, '/proc/self/fd/42');
+          expect(displayName, 'Song.flac');
+          return {
+            'replaygain_track_gain': '-6.00 dB',
+            'replaygain_album_gain': '-3.00 dB',
+          };
+        },
+      );
+      expect(
+        await cache.volumeFor(
+          '/proc/self/fd/42',
+          displayName: 'Song.flac',
+          cacheKey: 'content://music/song',
+        ),
+        closeTo(0.501187, 0.000001),
+      );
+      expect(
+        await cache.volumeFor(
+          '/proc/self/fd/99',
+          displayName: 'Song.flac',
+          cacheKey: 'content://music/song',
+        ),
+        closeTo(0.501187, 0.000001),
+      );
+      expect(reads, 1);
+    },
+  );
+
+  test(
+    'failed descriptor metadata does not poison local fallback cache',
+    () async {
+      var reads = 0;
+      final cache = PlaybackNormalizationCache(
+        readMetadata: (path, {displayName}) async {
+          reads++;
+          if (path.startsWith('/proc/')) {
+            return {'error': 'descriptor access denied'};
+          }
+          return {'replaygain_album_gain': '-6.00 dB'};
+        },
+      );
+      expect(
+        await cache.volumeFor(
+          '/proc/self/fd/42',
+          displayName: 'Song.mp3',
+          cacheKey: 'song',
+        ),
+        1.0,
+      );
+      expect(
+        await cache.volumeFor('/cache/Song.mp3', cacheKey: 'song'),
+        closeTo(0.501187, 0.000001),
+      );
+      expect(reads, 2);
+    },
+  );
+
+  test('exceptions are retried and positive gain only attenuates', () async {
+    var reads = 0;
+    final cache = PlaybackNormalizationCache(
+      readMetadata: (path, {displayName}) async {
+        if (++reads == 1) throw StateError('temporarily unavailable');
+        return {'replaygain_track_gain': '+6.00 dB'};
+      },
+    );
+    expect(await cache.volumeFor('song'), 1.0);
+    expect(await cache.volumeFor('song'), 1.0);
+    expect(reads, 2);
+  });
+}