Step 5. The requirement is "full configuration in the interface", and the trap is silent:
api_model_configure REBUILDS the entry from scratch and copies only an explicit
whitelist, so a key with a UI control but no whitelist entry is written once and deleted
on the next save from the models page. It looks saved until you reload. That is precisely
why H3's documented h3_venv / in_process / dtype keys never survive, and it is not
inherited here.

- `longcat` is now an option in the hard-coded cfg-backend select — without that the
  backend cannot be pinned from the UI at all, which is the other half of why H3's
  `backend: "h3"` switch is unreachable.
- A cfg-longcat field group, shown for that backend: variant (bf16/fp8/int8/gguf),
  default quality, offload, segments, conditioning frames, context-parallel GPUs, repo
  checkout, venv, and the avatar fields (reference frame index, mask range, INT8,
  distilled sampling, KV-cache offload). Populated on open and read back on save.
- All thirteen keys added to the whitelist, with a comment saying what happens without it.
- The settings now DO something: the dispatch reads num_segments, num_cond_frames,
  offload_kv_cache and quality off the entry as defaults, with the request still winning.
  A knob that is saved but never read is worse than no knob.
- docs/longcat-video.md: why the venv exists, the two prerequisites, the download repos
  and why file_pattern matters (~83 GB), the checkpoint layout, the stages and presets,
  the segment arithmetic, the VRAM/eviction/segment-boundary behaviour, and an explicit
  list of what is NOT done — including that no generation has been verified against real
  weights.

62 tests, parameterised over every field so a future control added without a whitelist
entry fails rather than silently losing data.

Not in this step: there is no known-model catalogue in the codebase to pre-register the
LongCat repos into — adding a model is operator-driven by model_id — so the repos and
their file patterns are documented instead of hardcoded.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY