feat(tts): Xiaomi MiMo per-beat voice + MOCK_IMAGE testing aid (#3)

Adds optional Xiaomi MiMo TTS layer on top of the scene/beat engine and a MOCK_IMAGE flag for cheap local TTS iteration.

- Per-character voice provisioning via MiMo voice design → clone, reference audio persisted in session
- Per-line free-form delivery direction (Director writes "鼓起勇气又害羞，声音发颤" style instructions; sent to MiMo's director channel, never read aloud)
- Per-beat audio served with the scene response; frontend plays via hidden <audio> with typewriter synced to audio duration; mute toggle persisted via localStorage lazy initializer
- Graceful degradation: any TTS step failing → silent beat, game continues
- MOCK_IMAGE=true returns a sharp-generated placeholder PNG so local TTS iteration doesn't burn image tokens
- Recommended config in .env.example: MiMo Token Plan covers TEXT/VISION/TTS with one key (mimo-v2.5-pro for text, mimo-v2.5 omni for vision, mimo-v2.5-tts for TTS)

Squashed from #3:
- feat(tts): 小米 MiMo 逐 beat 配音 + 按 session 角色音色 + 自由文本配音指导
- feat(engine): MOCK_IMAGE 占位图便于本地测试
- fix(tts): address Copilot review on PR #3
- fix(tts): Copilot round-2 review feedback

Known limitation: Session.characters carries the full WAV reference audio (~200-300KB/character base64) and round-trips through every /api/scene, /api/vision, /api/insert-beat request. This is intrinsic to MiMo's design→clone model (voice identity IS the audio, no server-side voiceId). Fixing requires server-side storage which is out of scope; documented for future hardening.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

apps/web/lib/config.ts

@@ -1,4 +1,4 @@
-import type { EngineConfig } from "@yume/types";
+import type { EngineConfig, TtsConfig } from "@yume/types";
 
 function readVar(name: string): string {
   const v = process.env[name];
@@ -6,6 +6,22 @@ function readVar(name: string): string {
   return v;
 }
 
+function readOptionalVar(name: string): string | undefined {
+  const v = process.env[name];
+  return v && v.length > 0 ? v : undefined;
+}
+
+function loadTtsConfig(): TtsConfig | undefined {
+  const baseUrl = readOptionalVar("TTS_BASE_URL");
+  const apiKey = readOptionalVar("TTS_API_KEY");
+  const speechModel = readOptionalVar("TTS_SPEECH_MODEL");
+
+  // Missing any → TTS disabled (game runs silently).
+  if (!baseUrl || !apiKey || !speechModel) return undefined;
+
+  return { baseUrl, apiKey, speechModel };
+}
+
 export function loadEngineConfig(): EngineConfig {
   return {
     text: {
@@ -23,5 +39,7 @@ export function loadEngineConfig(): EngineConfig {
       apiKey: readVar("VISION_API_KEY"),
       model: readVar("VISION_MODEL"),
     },
+    tts: loadTtsConfig(),
+    mockImage: readOptionalVar("MOCK_IMAGE") === "true",
   };
 }