When cross-backend pooling (offload.gpu_split) is on a model can span several
cards, so VRAM math must consider the pool, not one device:

- vulkan.py: new _pooled_free_vram_gb() sums free VRAM across every visible card
  (all CUDA devices + AMD via amdgpu sysfs; no double count). The GGUF auto-offload
  fit decision uses the pooled figure when the model is gpu_split, so layers aren't
  needlessly pushed to CPU just because one card's free VRAM looks small.
- manager._get_free_vram_gb(): when gpu_split is enabled, report pooled free across
  all GPUs (used by the eviction loop + budget checks), so eviction frees/measures
  capacity across both cards. Off → unchanged single-device behavior.

Pooling is aggregate (sum); strict per-device fitting/targeted eviction can be a
later refinement.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>