Multiple loaded models must each have their own queue — one model's
backlog or GPU ownership must never block requests for another model
that is loaded and idle:

- queue admission is per model (is_full_for): 429 only when the
  REQUEST'S model already has queue_max_size waiters (global 4×
  backstop bounds memory); previously ONE hot model's backlog 429'd
  every other model.
- GpuSwapGate: requests for a model already RESIDENT on the engine
  bypass the ownership gate — no swap is needed and its own per-model
  queue governs concurrency. The gate now serializes only requests
  that would actually trigger a model swap (its real purpose). Three
  co-resident embedders can each serve a request in parallel.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014S8VtAvG499SsCbeESRK7V