feat(zonos): OpenAI-ish REST adapter for asset-engine routing

Upstream Zonos ships only Gradio + Python SDK — no REST surface — so
asset-engine (which routes a clean JSON POST to /v1/audio/speech) can't
target it directly. Add a thin FastAPI adapter (stacks/zonos/adapter/):
POST /v1/audio/speech in front of the Zonos SDK, built FROM local/zonos
to reuse torch/CUDA/SDK. Returns a JSON envelope {audio, audio_format,
seed} — the seed rides back so asset-engine regenerate/fork can pin it
(Zonos is the fleet's first genuinely seedable TTS). compose gains a
zonos-api service on 8201; .env.example gains the port + voices dir.

stacks/zonos/adapter/requirements.txt

@@ -0,0 +1,6 @@
+# Adapter-only deps. torch / torchaudio / the zonos SDK already live in
+# the base image (local/zonos) — do NOT reinstall them here or the CUDA
+# build gets clobbered.
+fastapi==0.115.6
+uvicorn[standard]==0.34.0
+soundfile==0.12.1