Runs automatic platform discovery once, registering whatever host backends are available.

Overrides the context koblas returns; null restores automatic selection.

The process-wide default context: an installBackends override when set, else registered backends, else the portable reference implementations. Every free function in koblas uses this.

What this runtime resolved, for startup logging (e.g. "backend=openblas, kernels=simd(8 lanes)").

Offers backend as an explicit choice for every half it implements. Explicit registrations outrank automatically discovered ones; among explicit registrations, Backend.priority selects the winner.