Previously pooling was gated on gpu_split, so a 2-card same-backend engine (e.g.
2× 3090, or 2× Radeon) still measured only one device for the fit/eviction math.

Now both manager._get_free_vram_gb() and vulkan._pooled_free_vram_gb():
- ALWAYS sum every visible CUDA device (torch honours CUDA_VISIBLE_DEVICES, so it
  is scoped to this engine's NVIDIA cards) → same-backend split is accounted for
  with no flag.
- add AMD card(s) (amdgpu sysfs) only when cross-backend split is on OR no CUDA
  device is visible (a Radeon/Vulkan engine), so a Radeon engine counts its own
  cards and an NVIDIA engine only reaches across to Radeon when split is enabled.

So: 2× NVIDIA → summed across both NVIDIAs; 2× Radeon → summed across both Radeons;
split on → summed across all NVIDIA + Radeon.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>