feat: procedural season background with drifting petals

test/flutter_test_config.dart

@@ -0,0 +1,9 @@
+import 'dart:async';
+
+import 'package:block_seasons/ui/widgets/season_background.dart';
+
+Future<void> testExecutable(FutureOr<void> Function() testMain) async {
+  // Looping ambience animations never settle under pumpAndSettle.
+  debugDisableLoopingAnimations = true;
+  await testMain();
+}

test/ui/season_background_test.dart

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+import 'package:block_seasons/game/models/season.dart';
+import 'package:block_seasons/ui/widgets/season_background.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('renders and settles with looping animations disabled',
+      (tester) async {
+    await tester.pumpWidget(const MaterialApp(
+      home: SeasonBackground(theme: SeasonTheme.fallback),
+    ));
+    await tester.pumpAndSettle();
+    expect(find.byType(SeasonBackground), findsOneWidget);
+  });
+
+  testWidgets('particleType none still renders', (tester) async {
+    await tester.pumpWidget(const MaterialApp(
+      home: SeasonBackground(
+        theme: SeasonTheme(particleType: 'none'),
+      ),
+    ));
+    await tester.pumpAndSettle();
+    expect(find.byType(CustomPaint), findsWidgets);
+  });
+}