feat: wave-3 audio surfaces — stems, sfx, speech enhance, voice clone, translate (ADR-0024)

- audio.StemSeparator/StemSeparationProvider: Demucs zip transport via
  POST /upstream/<id>/v1/stems (Mode two -> two_stems=vocals; model +
  format fields); bounded zip unpack, entry name -> stem, ext -> MIME.
- SFXModel reuses musicgen against the sync /upstream/<id>/v1/sfx route
  (JSON prompt/seconds/steps/cfg_scale/seed -> WAV); musicgen.Request
  gains CFGScale.
- audio.SpeechEnhancer/SpeechEnhancementProvider:
  POST /upstream/<id>/v1/enhance -> WAV (result reuses SpeechResult).
- SpeechRequest.ReferenceAudio/ReferenceMIME (+WithReferenceAudio):
  llamaswap switches to the chatterbox clone route
  POST /upstream/<id>/v1/audio/speech/upload (input + voice_file),
  wav MIME fallback.
- TranscriptionRequest.Translate (+WithTranslate): translate=true form
  field, language=auto forced when no explicit hint (whisper.cpp default
  en would skip translation).
- httptest contract tests (zip unpack, clone-route switch, translate +
  auto-language injection); ADR-0024 (index row deferred — MJ-A backfills
  the ADR index table and parallel edits would conflict).

Co-Authored-By: Claude Fable 5 <[email protected]>

musicgen/musicgen.go

@@ -40,6 +40,11 @@ type Request struct {
 	// Steps is the number of inference steps; nil = backend default.
 	Steps *int
 
+	// CFGScale is the classifier-free-guidance scale; nil = backend default.
+	// Architecture-sensitive, so prefer leaving it nil unless the caller
+	// knows the target model. Backends without the knob ignore it.
+	CFGScale *float64
+
 	// Seed fixes the RNG seed for reproducible output; nil = backend
 	// default (random).
 	Seed *int64
@@ -73,6 +78,9 @@ func WithFormat(f string) Option { return func(r *Request) { r.Format = f } }
 // WithSteps overrides the number of inference steps.
 func WithSteps(n int) Option { return func(r *Request) { r.Steps = &n } }
 
+// WithCFGScale overrides the classifier-free-guidance scale.
+func WithCFGScale(s float64) Option { return func(r *Request) { r.CFGScale = &s } }
+
 // WithSeed fixes the RNG seed.
 func WithSeed(seed int64) Option { return func(r *Request) { r.Seed = &seed } }