The auto split is VRAM-proportional, which hands a big share to a slow second GPU
(e.g. RX 580) so it bottlenecks throughput (Radeon ~100%, 3090 ~30%). Add a
"performance" strategy that fills the fast lead card (main_gpu) first and spills
only the overflow to the slower card(s), using the model's expected size — so the
weak GPU holds the fewest layers (and none at all if the model fits on the lead
card). "vram" (default) keeps the capacity-maximizing behavior.

- config offload.split_strategy ("vram"|"performance") global default, persisted +
  pushed to live global_args; per-model split_strategy overrides it.
- plumbed via build_runtime_kwargs + manager _cfg_or_global into vulkan auto-split.
- model config modal: "Split strategy" select shown with the split options; global
  Settings: default split strategy select.
- split_strategy added to the live-reload (_LOAD_AFFECTING) set so changing it
  re-applies on the next request without a restart.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>