pick_engine honours the front's assignment (radeon) only if the engine
can_serve the request's required capability. But _required_cap derived
that capability from the bare alias 'coe-…-q4_k_m' — no literal 'gguf' —
so required_capability returned 'transformers' (CUDA-only). radeon is
gguf-only, failed can_serve, and the request fell through to the default
engine (nvidia), even though compute_assignment had correctly placed the
model on radeon (it sees the full '…-q4_k_m.gguf' path).

Resolve the model's configured path in _load_pins (now indexed by the
.gguf-stripped stem too) and, when the name heuristic yields
'transformers' but that path is a .gguf, correct the capability to
'gguf'. whisper/ds4 precedence is unchanged. Combined with the registry
stem-matching, a bare-alias request now lands on the owning Vulkan/AMD
engine.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>