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/tts-client/index.ts

@@ -1,15 +1,32 @@
-import type { CharacterVoice, TtsConfig } from "@infiplot/types";
-import { stepfunProvision, stepfunSynthesize } from "./stepfun";
+import type { CharacterVoice, TtsConfig, TtsProvider } from "@infiplot/types";
+import {
+  formatStepfunCatalogForPrompt,
+  isStepfun,
+  isValidStepfunVoiceId,
+  stepfunProvision,
+  type StepfunProvisionOptions,
+  stepfunSynthesize,
+} from "./stepfun";
 import { xiaomiProvision, xiaomiSynthesize } from "./xiaomi";
 
-// Provider auto-detection by base URL — mirrors the image client convention
-// of inferring Runware from *.runware.ai and falling back otherwise. Keeps
-// the BYO client flow unchanged: TTS_PROVIDER env var stays unused, and
-// browser-side keys (Xiaomi only today) keep working through the xiaomi path.
-function isStepfun(cfg: TtsConfig): boolean {
-  return /(^|[./])stepfun\.com\b/i.test(cfg.baseUrl);
+// Re-export so /api/tts-provider, orchestrator, CharacterDesigner prompt, and
+// the client all share ONE provider-detection rule + ONE catalog rendering +
+// ONE validity check with the synth path.
+export { isStepfun, isValidStepfunVoiceId, formatStepfunCatalogForPrompt };
+
+/** Map a configured TtsConfig to its provider tag. Single source of truth for
+ *  the inference rule (host contains stepfun.com → stepfun, else xiaomi) so
+ *  /api/tts-provider and resolveVoice can't drift when a third provider is
+ *  added. A PRESENT TtsConfig always maps to a concrete provider — `null`
+ *  (no TTS configured) is the caller's responsibility to handle separately. */
+export function inferTtsProvider(cfg: TtsConfig): Exclude<TtsProvider, null> {
+  return isStepfun(cfg) ? "stepfun" : "xiaomi";
 }
 
+// `opts.stepfunVoiceId` threads the CharacterDesigner's LLM-selected preset
+// down to stepfunProvision. Xiaomi ignores it. See StepfunProvisionOptions.
+export type ProvisionVoiceOptions = StepfunProvisionOptions;
+
 export async function provisionVoice(
   cfg: TtsConfig,
   description: string,
@@ -18,9 +35,10 @@ export async function provisionVoice(
   // clip per call regardless. Threading it through keeps the API uniform
   // and prevents archetype collisions on the StepFun path.
   salt?: string,
+  opts?: ProvisionVoiceOptions,
 ): Promise<CharacterVoice> {
   return isStepfun(cfg)
-    ? stepfunProvision(cfg, description, salt)
+    ? stepfunProvision(cfg, description, salt, opts)
     : xiaomiProvision(cfg, description);
 }