Step 3: make LongCat a citizen of the memory and swap machinery rather than a model that
merely runs. Three gaps had to close.

1. THE ESTIMATE. _get_model_used_vram_gb had no branch for a `longcat:` key, so it fell
   through to scanning the checkpoint — ~83 GB across variants, which would have asked
   the manager to evict the whole box on every request. Added beside the ds4/colibri/vllm
   early returns: measured if known, else the starting estimate.

2. THE MEASUREMENT. No official VRAM figure exists for LongCat and the real peak moves
   with the variant, the stage and the offload mode, so the constant is a starting point
   and the service reports what it actually reached. It has to come from inside the
   worker: the pipeline loads lazily on the first request, so a host-side free-VRAM delta
   would also be counting whatever else was loading on the card. torch's
   max_memory_reserved is this process's own arenas — what the card genuinely cannot give
   to anything else. The HIGH-WATER mark is kept, because a draft run touches far less
   than a 720p refinement and reserving the smaller figure would under-evict later, and
   it is keyed per config_id/variant so an fp8 sibling cannot inherit a bf16 measurement.

3. GIVING THE CARD BACK. register_external_vram_releaser with the documented
   fn(needed_gb) -> float contract (not voice_clone's zero-argument one, which raises
   TypeError inside eviction and is therefore never reclaimed). The drain matters more
   here than anywhere: acquire_stt_backend POPS the model pool, so _is_key_busy is
   permanently False for this model and Pass 1 of release_idle_vram would tear the
   subprocess down mid-generation and lose a multi-minute request. The releaser waits for
   in-flight work, bounded by longcat.evict_drain_timeout_s (300 s).

Also: video can now take a slot in the front per-model queue. Every acquire site was
guarded by `_task_kind(path) == "text"`, so max_instances on a video model did nothing at
the front and nothing bounded concurrent generations but the engine and the swap gate.
_queues_at_front() enables it for video when — and only when — the operator has set
max_instances: doing it unconditionally would change behaviour for every existing video
model and start returning 503 at queue_max_size, which is not something to impose on
deployments that never asked. Applied at all four sites (direct, keepalive, both
brokered), and the `is_text` local it replaced was left dead, so it is gone.

21 tests, including the one that matters most: an eviction arriving mid-generation waits
and the request still completes.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY