The per-model engine selector was confusing: "Auto" implied single-card (correct)
but the only split choice was a generic "All GPUs". Replace with hardware-named,
ordered choices:
  - "Auto (single GPU — by capability / free VRAM)"  → one card (unchanged)
  - "<engine> (single card)"                         → pin to that card
  - "Split — <engine> first"                         → pool across all GPUs with
    that engine's card leading (main_gpu / larger share). Generated per engine, so
    on a 3090+Radeon box you get "Split — nvidia first" and "Split — radeon first".
Saving a split option sets gpu_split + pins engine=<lead>; the weight-ratio field
(blank = auto by free VRAM) shows only for a split choice.

Make per-model split actually function:
- engine_supervisor: expose every GPU to engines (allow_cross) when the global
  toggle OR ANY model entry has gpu_split — otherwise the lead engine couldn't see
  the foreign card.
- vulkan.py: when an engine sees BOTH backends' cards but THIS model isn't a split,
  confine it to its own backend (zero the foreign devices in tensor_split) so it
  can't accidentally spread — while same-backend multi-card split still works. Split
  models pool across all visible devices (lead card = device 0 = main_gpu).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>