Three related fixes for cross-GPU split of a vision model at large context:

1) gpu_detect: when exposing GPUs for cross-split (allow_cross), include only REAL
   hardware Vulkan devices (nvidia/amd/intel) and EXCLUDE software rasterizers
   (llvmpipe/lavapipe/virtio = vendor "other"). They're CPU-backed with no real
   VRAM, slower than native CPU offload, and their presence skewed the device list
   so the tensor_split ratio didn't line up with the actual cards (and layers could
   land on a fake GPU).

2) vulkan auto tensor_split now reserves the mmproj's size + compute margin on
   main_gpu (the projector always loads there), so the proportional split doesn't
   fill main_gpu to the brim and abort when CLIP can't allocate
   (GGML_ASSERT(buffer) failed).

3) auto-offload (n_gpu_layers sizing) subtracts the same mmproj reserve from the
   POOLED free VRAM, so when model+KV+projector exceed BOTH cards combined it
   reduces GPU layers and spills to CPU gracefully instead of crashing.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>