feat(tts): add StepFun preset-voice provider, route by URL + voice tag

Add StepFun step-tts-mini / step-tts-2 / stepaudio-2.5-tts as an alternate
TTS provider alongside Xiaomi MiMo. Auto-detected from TTS_BASE_URL host
(contains `stepfun.com` → StepFun; otherwise → MiMo), mirroring how the
image client infers Runware from `*.runware.ai`.

CharacterVoice becomes a discriminated union on `provider`:
- xiaomi: { referenceAudioBase64, mimeType } — unchanged
- stepfun: { voiceId, model, mimeType } — preset voice ID + chosen model

Provision dispatches on the current cfg's base URL; synthesis dispatches
on the voice's own `provider` tag so a session with mixed voices (e.g. a
provider switch mid-development) routes each beat through the correct
protocol. xiaomiSynthesize now guards against being called with a non-
xiaomi voice, surfacing the bug as a clear runtime error instead of a
TypeScript narrow violation at the access site.

StepFun has no voicedesign equivalent — only preset voices + voice
cloning from a reference audio upload. Cloning would require an extra
asset per character, so v1 maps the LLM's Chinese voiceDescription to one
of the 32 published preset IDs via gender + age + tone keyword scoring,
with a deterministic hash spread across the top-3 candidates so multiple
characters with similar descriptions don't collapse onto the identical
preset. lineDelivery is accepted but not yet propagated to StepFun's
voice_label.emotion / .style fields — left as a follow-up.

beat-audio route validation relaxed from `voice.referenceAudioBase64`
(xiaomi-shaped) to `voice.provider` (shape-agnostic), so stepfun voices
pass the gate; provider-specific shape errors still surface from the
synth function.

Observed latency on InfiPlot's dev loop: StepFun step-tts-mini median
~2.3s per beat with 0% timeouts across the test session, vs MiMo's
median ~8s with the long tail tripping the existing 15s synth budget
on roughly 2 of 3 beats. Pricing: step-tts-mini ¥0.9/万字符 (~¥0.14
per typical 50-beat session) vs MiMo TTS currently free under the
Token Plan creator incentive.

AGENTS.md provider matrix updated to describe both providers and the
discriminated-union dispatch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AGENTS.md

@@ -138,7 +138,7 @@ Use `.env.example` as the source of truth. Never commit `.env.local`, API keys,
 
 - Text and Vision use `TEXT_*` and `VISION_*`; default protocol is `openai_compatible`, with native `anthropic` and `google` available via `TEXT_PROVIDER` / `VISION_PROVIDER`.
 - Image uses `IMAGE_*`; supported protocols are `runware`, `openai_compatible`, native `openai`, and native `google`. When `IMAGE_PROVIDER` is unset, Runware is inferred from `*.runware.ai` URLs and otherwise falls back to OpenAI-compatible image generations.
-- TTS uses Xiaomi MiMo protocol and is optional: blank config means silent mode.
+- TTS supports Xiaomi MiMo (voicedesign + voiceclone) or StepFun (preset voices auto-selected by keyword scoring), inferred from `TTS_BASE_URL` (host containing `stepfun.com` → StepFun, otherwise → MiMo). `CharacterVoice` is a discriminated union on `provider`; synth dispatches on the voice's own tag so a session may carry both shapes through a provider switch. Blank config means silent mode.
 - `MOCK_IMAGE=true` skips image generation and returns a placeholder for cheap local iteration.
 - `NEXT_PUBLIC_IMAGE_PROXY_URL` and `NEXT_PUBLIC_IMAGE_PROXY_ALLOWED_HOSTS` opt into browser-side image proxying for allowed hosts.
 - Analytics uses optional Umami `NEXT_PUBLIC_UMAMI_*` values and must stay content-free/privacy-preserving.