Function resolve_budget 

const fn resolve_budget(mem: &SystemMemoryInfo) -> (u64, BudgetSource)

Resolve the memory figure to size against, and say where it came from.

VRAM wins when it is known, because that is the memory the weights will actually occupy. It is None on every Vulkan-only machine — gglib reads VRAM for Metal and NVIDIA only — so an AMD or Intel GPU falls back to host RAM. That fallback is usually too generous, which is exactly why BudgetSource is returned alongside the number instead of being thrown away: the caller is expected to say so.