whisper.cpp bakes in one GPU backend at compile time, so a BackendId alone
can't say how a vendor is served. Add AccelPath + resolve_accel(_with) as the
single source of truth over (compiled features x runtime readiness), and derive
BackendInfo.available from it in the DXGI + NPU paths. Closes the gap where a
present-but-unsupported GPU was marked available and best() routed to a no-op
GPU. Pure resolve_accel_with is unit-tested across all vendor/feature combos.
iamdoubz
e9567b823a