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/prompts.ts

@@ -7,6 +7,7 @@ import type {
   StoryState,
   WriterPlan,
 } from "@infiplot/types";
+import { formatStepfunCatalogForPrompt } from "@infiplot/tts-client";
 
 // ══════════════════════════════════════════════════════════════════════
 //  Multi-agent scene generation pipeline:
@@ -599,7 +600,14 @@ function collectPriorSceneKeys(session: Session): string[] {
 //  (e.g., gentle-looking character with energetic voice).
 // ──────────────────────────────────────────────────────────────────────
 
-export const CHARACTER_DESIGNER_SYSTEM = `你是视觉小说的「角色设定师」。给你一个**新登场角色的名字**，你要为这个角色同时设计两份卡片：
+// CHARACTER_DESIGNER_SYSTEM is split into a provider-agnostic CORE (visual +
+// voice-text rules) and a provider-specific TAIL (the JSON contract). When the
+// server runs StepFun, the tail additionally asks the model to pick a preset
+// voice id from the 32-entry catalog — so the SAME LLM call that designs the
+// character also selects its voice, at zero extra latency. When StepFun is
+// off (Xiaomi / no TTS), the tail is byte-identical to the historical prompt
+// (Xiaomi path is cache- and behavior-preserving).
+const CHARACTER_DESIGNER_SYSTEM_CORE = `你是视觉小说的「角色设定师」。给你一个**新登场角色的名字**，你要为这个角色同时设计两份卡片：
 1. **视觉设定卡（英文）**——给生图模型 FLUX 用，遵循 prompt engineering 风格
 2. **音色设定卡（中文）**——给小米 MiMo 配音设计用
 
@@ -652,7 +660,12 @@ export const CHARACTER_DESIGNER_SYSTEM = `你是视觉小说的「角色设定
 - 随后描述：年龄段（如「约17岁少女」「30 出头男性」）、音色质感、性格情绪基调、语速节奏、人设腔调、口音方言
 - 用中文，整段连续描述，不分段
 - 长度：50–80 个中文字为宜
-- 例："女性，约17岁少女，音色清亮带点稚嫩甜美，性格开朗外向但容易害羞，语速偏快，标准普通话"
+- 例："女性，约17岁少女，音色清亮带点稚嫩甜美，性格开朗外向但容易害羞，语速偏快，标准普通话"`;
+
+// JSON-contract tail for the NON-stepfun path (Xiaomi voicedesign / no TTS).
+// Byte-identical to the historical prompt so the Xiaomi path keeps its cache
+// hit rate and voice quality unchanged.
+const CHARACTER_DESIGNER_TAIL_DEFAULT = `
 
 必须输出严格 JSON：
 {
@@ -662,6 +675,43 @@ export const CHARACTER_DESIGNER_SYSTEM = `你是视觉小说的「角色设定
 
 不要输出 JSON 以外的任何文本。`;
 
+// JSON-contract tail for the StepFun path. Same core output, plus the model
+// picks a preset voice id from the catalog. The id must match the SAME person
+// the voiceDescription describes (gender / age / vibe) — designed together so
+// appearance and voice stay coherent (the same invariant the CORE enforces).
+const CHARACTER_DESIGNER_TAIL_STEPFUN = `
+
+**StepFun 预设音色选择（必做）：**
+除 voiceDescription 外，你还必须从下列 StepFun 预设音色清单中，为本角色挑选一个与 voiceDescription 描绘的「同一个人」（性别 / 年龄段 / 气质都要一致）最贴合的预设，并把它的 id 填入 stepfunVoiceId。清单：
+${formatStepfunCatalogForPrompt()}
+
+挑选原则：
+- stepfunVoiceId 必须是上表里某个 id，原样复制（拼写、大小写、连字符都不能变）。
+- 必须与 voiceDescription 的性别一致（男声选 male 行，女声选 female 行）。
+- 年龄段尽量一致；拿不准时优先气质匹配（例如“冷艳御姐”选 lengyanyujie、“软萌萝莉”选 ruanmengnvsheng）。
+- 不允许编造清单外的 id，也不允许留空。
+
+必须输出严格 JSON：
+{
+  "visualDescription": "English visual card, comma-separated tags...",
+  "voiceDescription": "中文音色卡，以性别开头...",
+  "stepfunVoiceId": "清单内某个 id"
+}
+
+不要输出 JSON 以外的任何文本。`;
+
+/** Build the CharacterDesigner system prompt, provider-aware.
+ *  - stepfun:false → identical to the historical Xiaomi/no-TTS prompt.
+ *  - stepfun:true  → additionally asks the model to pick a StepFun preset
+ *    voice id from the 32-entry catalog (see formatStepfunCatalogForPrompt). */
+export function buildCharacterDesignerSystem(opts: {
+  stepfun: boolean;
+}): string {
+  return opts.stepfun
+    ? CHARACTER_DESIGNER_SYSTEM_CORE + CHARACTER_DESIGNER_TAIL_STEPFUN
+    : CHARACTER_DESIGNER_SYSTEM_CORE + CHARACTER_DESIGNER_TAIL_DEFAULT;
+}
+
 export function buildCharacterDesignerUserMessage(
   charName: string,
   session: Session,