feat: extend SeasonTheme with visual identity fields (ARGB ints)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/game/models/season_test.dart

@@ -41,7 +41,24 @@ void main() {
 
     test('round-trips to JSON', () {
       final pack = SeasonPack.fromJson(packJson);
-      expect(pack.toJson(), packJson);
+      // toJson() always emits all SeasonTheme fields (new fields added in
+      // Task 1). Compare everything except theme separately so that adding
+      // more theme fields in the future only requires updating theme tests.
+      final json = pack.toJson();
+      expect(json['schemaVersion'], packJson['schemaVersion']);
+      expect(json['seasonId'], packJson['seasonId']);
+      expect(json['version'], packJson['version']);
+      expect(json['title'], packJson['title']);
+      expect(json['stages'], packJson['stages']);
+      // Theme: legacy fields preserved, new fields present with defaults.
+      final theme = json['theme'] as Map<String, dynamic>;
+      expect(theme['tileSet'], 'spring');
+      expect(theme['background'], 'background.webp');
+      expect(theme['backgroundGradient'], SeasonTheme.defaultGradient);
+      expect(theme['accentColor'], 0xFFFF7EB3);
+      expect(theme['particleType'], 'petals');
+      expect(theme.containsKey('tilePalette'), isFalse);
+      expect(theme.containsKey('boardTint'), isFalse);
     });
 
     test('localized title falls back to English', () {
@@ -58,4 +75,42 @@ void main() {
       );
     });
   });
+
+  group('SeasonTheme visuals', () {
+    test('legacy theme json (tileSet/background only) gets defaults', () {
+      final theme = SeasonTheme.fromJson({
+        'tileSet': 'spring',
+        'background': 'background.webp',
+      });
+      expect(theme.backgroundGradient, SeasonTheme.defaultGradient);
+      expect(theme.accentColor, 0xFFFF7EB3);
+      expect(theme.particleType, 'petals');
+      expect(theme.tilePalette, isNull);
+      expect(theme.boardTint, isNull);
+    });
+
+    test('full theme json round-trips', () {
+      final theme = SeasonTheme(
+        tileSet: 'summer',
+        background: 'bg.webp',
+        backgroundGradient: const [0xFF0A2430, 0xFF10394A, 0xFF1E5A66],
+        accentColor: 0xFF6FCDF5,
+        particleType: 'snow',
+        tilePalette: const [0xFF111111, 0xFF222222],
+        boardTint: 0xFF041016,
+      );
+      final decoded = SeasonTheme.fromJson(theme.toJson());
+      expect(decoded.backgroundGradient, theme.backgroundGradient);
+      expect(decoded.accentColor, theme.accentColor);
+      expect(decoded.particleType, theme.particleType);
+      expect(decoded.tilePalette, theme.tilePalette);
+      expect(decoded.boardTint, theme.boardTint);
+    });
+
+    test('fallback constant matches season 1 defaults', () {
+      expect(SeasonTheme.fallback.backgroundGradient,
+          const [0xFF0E1430, 0xFF16204A, 0xFF2A2E5E]);
+      expect(SeasonTheme.fallback.particleType, 'petals');
+    });
+  });
 }