feat: season 2 'Summer Tide' content and manifest generator

Adds Summer Tide (season_002) — 30 stages with deep-teal gradient and
cyan accent — generated remote-only (no copyToAssets). Introduces
tool/make_manifest.dart to rebuild content/manifest.json from all
season packs with SHA-256 verification support.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tool/make_manifest.dart

@@ -0,0 +1,41 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:crypto/crypto.dart';
+
+/// Builds content/manifest.json from the season packs under content/.
+/// Usage: dart run tool/make_manifest.dart
+void main() {
+  final contentDir = Directory('content');
+  final seasons = <Map<String, dynamic>>[];
+
+  final dirs = contentDir
+      .listSync()
+      .whereType<Directory>()
+      .where((d) => File('${d.path}/pack.json').existsSync())
+      .toList()
+    ..sort((a, b) => a.path.compareTo(b.path));
+
+  for (final dir in dirs) {
+    final file = File('${dir.path}/pack.json');
+    final bytes = file.readAsBytesSync();
+    final pack = jsonDecode(utf8.decode(bytes)) as Map<String, dynamic>;
+    seasons.add({
+      'seasonId': pack['seasonId'],
+      'version': pack['version'],
+      'packUrl': 'seasons/${pack['seasonId']}/pack.json',
+      'sha256': sha256.convert(bytes).toString(),
+    });
+  }
+
+  final manifest = {
+    'schemaVersion': 1,
+    'minAppBuild': 1,
+    'current': seasons.isEmpty ? '' : seasons.last['seasonId'],
+    'seasons': seasons,
+  };
+
+  File('content/manifest.json').writeAsStringSync(
+      '${const JsonEncoder.withIndent("  ").convert(manifest)}\n');
+  stdout.writeln('manifest.json written with ${seasons.length} season(s).');
+}