Step 4. It also CORRECTS step 2, whose generate() was written against an assumed
interface. The real one, read from upstream's demo scripts, differs in four ways that all
mattered:

- there is no single callable: generate_t2v(height, width, …), generate_i2v(image,
  resolution, …), generate_vc(video, num_cond_frames, use_kv_cache, offload_kv_cache, …)
  and generate_refine(stage1_video, spatial_refine_only, …) are separate methods, and
  t2v takes height/width where i2v and vc take a resolution NAME;
- each returns [0]-indexed frames normalised to [0,1], not an object with .frames —
  upstream converts with (tensor * 255).clamp(0,255).to(uint8);
- the distilled stage is a use_distill=True FLAG on the same method, not a LoRA to fuse
  by hand, so _apply_stage_lora is gone;
- the 50-step passes take a negative_prompt and the distilled one does not (its guidance
  is 1.0, so it would do nothing).

On top of that:
- The segment loop is upstream's: a call emits num_frames but its first num_cond_frames
  re-render the previous tail, so only the remainder is new and the fresh frames become
  the next call's conditioning. torch_gc() between segments, as upstream does.
- Modes map onto the tasks: t2v, i2v/ti2v -> i2v, extend/vc/v2v -> vc. Continuation
  reuses the EXISTING cond_frames field, so a client that already chains clips through
  the VACE path needs no new vocabulary — and gets native pretrained continuation
  instead, which is what stops the colour drift chained conditioning suffers.
- Long video: num_segments or duration_seconds, and the SERVER owns the loop. The
  response carries the tail a caller needs to continue by hand if it prefers to.
- The segment boundary is now a real handover point. POST /yield asks a generation to
  stop at the next boundary; the releaser calls it before waiting, so an eviction
  arriving mid-request costs one segment instead of the whole generation, and the request
  still returns — shorter, with a warning that reaches the caller — rather than failing.

Unsupported modes (the audio-driven avatar tasks) are still refused with a 400 rather
than downgraded to t2v, which would read as a quality bug rather than a gap.

23 new tests pinning the upstream call contract so it cannot drift back, plus the segment
arithmetic and the yield behaviour. Still no real generation: that needs the venv, the
repo checkout and the ~83 GB checkpoint.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY