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>

content/season_002/spec.json

@@ -0,0 +1,15 @@
+{
+  "seasonId": "season_002",
+  "version": 1,
+  "title": { "en": "Summer Tide", "ko": "여름 파도" },
+  "theme": {
+    "tileSet": "summer",
+    "background": "",
+    "backgroundGradient": [4278854704, 4279253322, 4280179302],
+    "accentColor": 4285517301,
+    "particleType": "petals"
+  },
+  "stageCount": 30,
+  "baseSeed": 20260612,
+  "runsPerStage": 80
+}