write songs with a model the server cannot hold in its own interpreter

ACE-Step generates a whole track -- style prompt, sung lyrics, a length in
seconds -- and it ships a package pinning transformers 4.50 and spacy 3.8.4.
spacy 3.8.4 publishes no cp313 wheel, so this is not a stack the server would
rather not hold: it is one that cannot be installed on the server's
interpreter. It gets a python 3.12 venv of its own and an HTTP service in it,
driven the way CrisperWhisper, Canary and pyannote are. Built on the first
request to such a model, so a model that is configured and never called costs
nothing; `uv venv` covers the hosts that have python3.12 without ensurepip.

Three upstream details that fail silently rather than loudly:

* the pipeline resolves its own dtype as `bfloat16 if dtype == "bfloat16" else
  float32`, so asking it for float16 quietly doubles the VRAM. ACE_PIPELINE_DTYPE
  is read after that switch, so precision goes through there instead.
* constructing the pipeline loads NOTHING -- it reads the checkpoints on the
  first call. coderai measures a model's footprint as the free-VRAM delta
  across the load, and that window closes when the worker reports healthy, so
  a lazy load would measure ~0 GB and eviction would treat a resident 3.5B
  model as free. The service loads before answering /health.
* `quantized=True` does not quantize: it loads a separate int4wo export from
  the same directory layout. Missing, that is a FileNotFoundError minutes into
  a load, so the worker names the file it wants first.

The handle cached in the manager exposes cleanup(), which is what eviction
calls, and here it stops the worker -- the only thing that frees its VRAM.

`duration` and `cfg_coef` are shared with MusicGen and default to 10s/3.0,
which would make every ACE-Step call a 10-second clip at a guidance it is not
tuned for. The branch reads which fields the client actually SENT, so an
omitted one gets ACE-Step's default and an explicit `duration: 10` still means
10. `lyrics` and `steps` become declared fields: both branches already read
`steps` off the request, where only extra="allow" was letting it through.

The venv is ~6.4 GB, so the default image does not carry it and --with-acestep
bakes it in -- copying the dev venv when there is one, building it when there
is not. Ubuntu 22.04's system python is 3.10, so a baked 3.12 venv also needs
a relocatable interpreter bundled beside it, like the 3.10 venvs.

Verified on a 3090: healthy after ~3.5 min, 8.5 GB VRAM, a real 48 kHz stereo
WAV from a 10s/12-step request. Suite at its 15-failure baseline, 5292 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu