Downloads: run each model download in a clean `python -m
codai.admin.download_worker` subprocess streaming JSON progress, so the
Stop button reliably cancels by terminating the process (HF parallel/Xet
chunk transfers ignore in-thread flags). Adds download-cancel-all. Avoids
multiprocessing spawn, which re-imports the server launcher as __main__.

VACE extension: detect WanVACEPipeline; new 'extend' mode + cond_frames
request field condition on the previous chained part's frame tail (real
motion -> forward continuation, fixing the single-frame boomerang).
_build_vace_conditioning builds the (video, mask) pair; _snap_wan_frames
enforces 4k+1; only the freshly generated frames are returned. VACE also
serves keyframe i2v / t2v via masking; i2v/t2v fallbacks skipped for it.
Township auto-uses extend for chained parts when the model is VACE.

Fight prompts: full-MMA system prompt + rotating per-clip action focus
(kicks/knees/elbows/takedowns/ground/submissions) and occasional blood,
rebalanced fallback templates, keyframe wardrobe enforcement.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>