Task #8. Question: is a SOTA Mel-/BS-Band Roformer reachable on the 6GB RTX 2060,
or are we stuck on demucs? Answer: it fits comfortably — but the wall was never VRAM.

Run 1 (system python 3.14) died at model instantiation: audio-separator's Roformer
loader carries `Callable | None` type hints that beartype rejects under 3.14
(BeartypeDecorHintNonpepException on `stft_window_fn`). Peak VRAM at crash: 921 MiB —
nowhere near the limit. So engine2 is gated on the interpreter, not the card.

Run 2 (python 3.12 venv) succeeded: bs_roformer_ep_317 separated the 164s Loituma
source in 90s (~0.55× realtime) at a peak of 3051/6144 MiB — half the card, ~3GB
headroom. torch 2.12.1+cu130, onnxruntime-gpu 1.27, CUDA detected fine.

Caveat for the wiring task (#9): ep_317 is a 2-stem model (Instrumental/Vocals); the
Foundry contract is 4 stems (drums/bass/other/vocals). #9 must pick a 4-stem Roformer
checkpoint and re-confirm VRAM — but with 3GB to spare on a 2-stem pass, a 4-stem run
is plausible. The spike is self-contained + idempotent (research/engine2_spike/spike.sh,
SPIKE_PY pins the interpreter); result.json carries the numbers.