v1.0.8

v1.0.8 — UVR phantom-output root cause + remix stems arg

The "model produced no output files" failure that bit every
downstream consumer of /v1/audio/restore/uvr-* turned out to be a
3-layer config bug, not a model issue:

1. audio-separator snapshots output_dir at load_model() time;
   mutating sep.output_dir after load only updates the outer
   wrapper. Inner model_instance still writes to cwd (/app/,
   root-owned) → EACCES → library silently catches → audiolla sees
   "claimed but never written" files and drops them all.
2. output_single_stem filter matched the wrong stem-name namespace
   (model's own primary_stem_name vs engines.json's primary_stem).
3. Near-silent stem dropper masked layers 1 and 2 on synthetic
   fixtures.

Fix: drop output_single_stem= from the Separator() call (audiolla
picks the right stem post-hoc); propagate output_dir to
sep.model_instance.output_dir alongside sep.output_dir before each
separate() call. Plus a heavy-reverb test fixture + strict UVR
output assertions (>= 10 s real WAV, no graceful no-output skip).

Also: remix handler now passes stems to separate() (was failing
with TypeError, masked pre-v1.0.7 by FastAPI's plain-text 500).

482/482 CUDA integration tests passing.