feat(tts): StepFun voice selection via CharacterDesigner + provider-aware beat-audio

Make homepage cards and live sessions produce sound when the server is
configured for StepFun TTS, instead of silently failing (the prebaked
Xiaomi voice was useless on a StepFun server, and wasted ~220KB/beat in
Fast Origin Transfer).

Three coordinated changes:

1. CharacterDesigner now picks a StepFun preset voice id directly from the
   32-entry catalog in the SAME LLM call that designs the character — zero
   extra latency, LLM-grade match quality. The Xiaomi prompt path is
   byte-identical to history (verified programmatically) so cache hit rate
   and voice quality are preserved. pickStepfunVoiceId (keyword scorer)
   remains the fallback for orphan speakers / invalid LLM picks.

2. The 32-preset catalog moves to lib/tts-client/stepfun-voices.json as the
   single source of truth, shared by the scorer, the CharacterDesigner
   prompt, /api/tts-provider, and the offline enrich script.

3. A new GET /api/tts-provider endpoint lets the client probe the server's
   TTS provider at /play mount. fetchBeatAudio then shapes its request body:
   on a StepFun server it sends the lightweight stepfunVoiceId /
   voiceDescription and omits the ~220KB Xiaomi reference audio (FOT saving
   ~13MB per protagonist per session on prebaked cards). requestBeatAudio
   re-provisions on a provider mismatch before synth, so audio never goes
   silent on a cross-provider replay or mid-session provider flip.

New type fields are all optional and backward-compatible: Character.stepfunVoiceId,
BeatAudioRequest.voiceDescription/characterName/stepfunVoiceId, voice made
optional. AGENTS.md updated for the new route, type fields, dependency map,
and StepFun voice-selection flow.

lib/engine/director.ts

@@ -305,8 +305,13 @@ export async function directScene(
   }
 
   // Kick off voice provisioning for every NEW char (never on the paint path).
+  // On the StepFun path, thread the LLM-selected stepfunVoiceId from the card
+  // into provision — it lets stepfunProvision honor the catalog pick instead
+  // of falling back to the keyword scorer (same network cost: still zero).
   const voicePromises = cards.map((card) =>
-    provisionCharacterVoice(config, card.voiceDescription, card.name).then(
+    provisionCharacterVoice(config, card.voiceDescription, card.name, {
+      stepfunVoiceId: card.stepfunVoiceId,
+    }).then(
       (voice): Character => ({
         name: card.name,
         voiceDescription: card.voiceDescription,