Found while investigating why breadpad's test suite went from fast (silent
CPU-only ROCm no-op) to multi-minutes slow after fixing the ROCm->MIGraphX
bug in the previous commit: MIGraphX has no Rust-level cache-directory
builder (unlike OpenVINO/Vitis) — it's controlled purely by the
ORT_MIGRAPHX_MODEL_CACHE_PATH env var, read by the underlying MIGraphX
library. Left unset, every session build recompiles every kernel from
scratch with no persistence at all, visible in breadpad's test output as
repeated `migraphx_save: Error: ... write_buffer: Failure opening file:
""/<hash>.mxr` lines (empty path prefix = var never set).
breadmill's own packaged systemd unit already sets this explicitly
(packaging/breadmill.service: ORT_MIGRAPHX_MODEL_CACHE_PATH=%h/.cache/
breadsearch/migraphx-cache) but nothing enforced any other consumer doing
the same — breadpad had no systemd unit or cache path at all. Provider::
MiGraphX's to_dispatch now defaults ORT_MIGRAPHX_MODEL_CACHE_PATH to
~/.cache/bread-onnx/migraphx if unset, so every consumer gets kernel-cache
persistence for free instead of only the ones that remembered to
configure it themselves.
49b384c549