Belt-and-suspenders for the "untracked in-use VRAM" failure class (a teardown that
drops a model's registry entry but leaves GPU tensors resident — e.g. activations
pinned by an exception traceback):

- manager.sweep_orphan_vram(): gc.collect() + empty_cache() reclaims the
  UNREFERENCED kind (a just-dropped-but-not-collected pipeline), and logs a
  diagnostic when the card holds materially more VRAM than tracked models account
  for (the referenced kind, which can't be freed here — must be fixed at source).
- manager._estimate_tracked_vram_gb(): sums tracked models' footprints for that
  check (offloaded models over-report, so the check is conservative, never a false
  positive on a legitimately offloaded model).
- video: call it at the start of every load — right after request_model's eviction
  — so a model swap is verified to have actually freed the outgoing model's VRAM,
  and any stray orphan is swept before the new model loads.

Same-model consecutive requests already reuse the resident (offloaded) pipe via
request_model, so this only runs on a genuine (re)load.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce