Add a cap on how much VRAM the auto-split may place on EACH secondary (non-main)
card, so a slow second GPU (e.g. RX 580) stays lightly loaded and bottlenecks
throughput less — the remainder stays on the fast card or spills to CPU.

- config offload.split_secondary_cap_gb: global default, persisted + pushed live to
  every engine's global_args (each engine reads config at startup, so it's global).
- per-model split_secondary_cap_gb overrides — but only to TIGHTEN: the effective
  cap is min(global, per-model) of whichever are set; a per-model value higher than
  the global is ignored.
- applied in vulkan auto-split: caps each non-main device's free VRAM in the ratio
  (both vram & performance strategies), and in the auto-offload pool so the excess
  proactively spills to CPU instead of only reacting to an OOM.
- UI: model config modal "Secondary card VRAM cap" + global Settings field.
- added to _LOAD_AFFECTING so changes re-apply live on the next request.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>