Add the MIGraphX rung for AMD GPUs

Measured on a Radeon RX 7900 XT against Arch's onnxruntime-rocm 1.29
(docs/inference.md §1.3): MIGraphX fp16 runs the detectors at 2.4–3.4 ms
against 10–58 ms on the CPU provider, the inpainter at 8 ms against 514,
with a 15–135 s compile per graph the first time and under a second from
its cache after. A compiling rung on TensorRT's terms, wired the same way.

The ROCm execution provider is gone (removed in ONNX Runtime 1.23), so the
AMD ladder is MIGraphX then the CPU, with no non-compiling rung between.

MIGraphX is registered through the runtime's generic key/value entry
point rather than ort's builder: 1.29 reads the legacy options struct for
its precision flags only, and the compiled-program cache directory
(`migraphx_model_cache_dir`) only travels the generic way. The provider's
cache key omits the precision, so f32 and fp16 programs get their own
directories. The probe fingerprint now includes the provider libraries
beside the runtime and the ROCm version, since a distribution's CPU and
ROCm builds are the same file at the same path.

`status().failed` reports only the rungs above the selection, so an AMD
desktop's About line says why MIGraphX won rather than that the NVIDIA
providers are not in the build.

Two examples: `ep_probe` times each provider cold and from cache, and
`ladder` drives `init` as the app does to watch the first-run sequence.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/fetch-desktop-runtime.sh

@@ -13,6 +13,11 @@
 # which is what this script exists to fix. Nothing NVIDIA is bundled here:
 # the providers load CUDA, cuDNN and TensorRT from the system, and if those
 # are missing the probe says so and the app stays on the CPU.
+#
+# This is the NVIDIA script. On AMD there is nothing to fetch: the
+# distribution's ROCm build of ONNX Runtime (Arch's `onnxruntime-rocm`)
+# carries the MIGraphX provider, and the app finds it in the system library
+# directory (docs/inference.md §1.3).
 set -euo pipefail
 DEST="${1:-${XDG_DATA_HOME:-${HOME}/.local/share}/darkroom/runtime}"
 WORK="$(mktemp -d -p /var/tmp fetch-desktop-runtime.XXXXXX)"