Make unreclaimable VRAM actionable, and account for ComfyUI's CUDA context

The health check now reports what unmanaged VRAM actually costs rather than just
how much of it there is: "0.82 GB held by python (842 MB)" becomes "5 model(s) fit
within 15.42 GB but not the 14.60 GB actually available", naming them.

Getting that arithmetic right took a correction. The first version subtracted only
the desktop and the unmanaged process, and so reported a 14.93 GB model as fitting
against a real ceiling of 14.60 GB -- the same model the service had just refused
with 507. ComfyUI keeps a few hundred MB of CUDA context for as long as the process
lives, which a purge does not free, so it is not available either. The floor is taken
from the minimum ComfyUI VRAM in recent telemetry rather than its current value,
which could be a 7 GB checkpoint mid-generation.

The verifier's reclaim stage now re-runs a graph immediately beforehand to reset the
30 s idle window, since a large model takes longer than that to load and the purge
was freeing ComfyUI mid-load, so the reclaim path was never reached.

Tests: 199 (was 192). The new ones pin the ceiling arithmetic, including that a model
too large to fit on the card at all is not blamed on the third-party process.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

verify_arbitration.py

@@ -215,6 +215,16 @@ async def stage_reclaim(c: httpx.AsyncClient, model: str) -> bool:
                f"HyperSwap cannot free")
         return True
 
+    # Re-run a graph first. The idle purge fires 30 s after ComfyUI goes quiet, and a
+    # large model takes longer than that to load -- so without resetting the timer the
+    # purge frees ComfyUI mid-load and the reclaim path is never reached.
+    sys.path.insert(0, "/home/drjones/unified-model-manager")
+    import autotune  # noqa: E402
+    await autotune._diffusion_benchmark()
+    gpu = await api(c, "GET", "/api/gpu")
+    print(f"       reset the idle window; ComfyUI holds "
+          f"{gpu['breakdown']['comfyui_gb']} GB, {gpu['vram_free_gb']} GB free")
+
     res = await api(c, "POST", "/api/switch-model", allow_error=True,
                     json={"model": model, "keep_alive": "2m"}, timeout=600)
     if res.get("_status") == 507: