Search the user's own runtime directory before the system library

The reference desktop's only system ONNX Runtime is Arch's
onnxruntime-opt-cuda: 1.29, built without TensorRT and against cuDNN 8
on a cuDNN 9 machine. The probe rejects both providers correctly and
the app runs on the CPU provider, which is right and not what anyone
wants. runtime/ beside the models is now searched ahead of /usr/lib,
tools/fetch-desktop-runtime.sh fills it with the four libraries from
the current onnxruntime-gpu wheel (cuDNN 9, TensorRT 10), and the
About caption lists every rung that lost and why, not only the first.
Verified: the app selects TensorRT from that directory with no
environment variable set.

tools/fetch-desktop-runtime.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Put a GPU-capable ONNX Runtime where the desktop app looks for one
+# (docs/inference.md §3): `runtime/` beside the models in the user data
+# directory, ahead of the system library.
+#
+#   ./tools/fetch-desktop-runtime.sh [DEST]
+#
+# The source is the `onnxruntime-gpu` wheel: the one build that carries the
+# CUDA *and* TensorRT providers against the cuDNN and TensorRT majors current
+# on this machine. Distribution packages tend to have neither — Arch's
+# `onnxruntime-opt-cuda` is built without TensorRT and against cuDNN 8 — and
+# the probe rejects them correctly and leaves the app on the CPU provider,
+# 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.
+set -euo pipefail
+DEST="${1:-${XDG_DATA_HOME:-${HOME}/.local/share}/darkroom/runtime}"
+WORK="$(mktemp -d -p /var/tmp fetch-desktop-runtime.XXXXXX)"
+trap 'rm -rf "${WORK}"' EXIT
+
+echo "==> downloading the onnxruntime-gpu wheel"
+uv venv --python 3.12 "${WORK}/venv" >/dev/null
+VIRTUAL_ENV="${WORK}/venv" uv pip install --quiet onnxruntime-gpu
+CAPI="$(find "${WORK}/venv" -type d -path '*/onnxruntime/capi' | head -1)"
+[[ -n "${CAPI}" ]] || { echo "error: no capi directory in the wheel" >&2; exit 1; }
+
+mkdir -p "${DEST}"
+# The runtime and its provider libraries; not the Python binding.
+cp "${CAPI}"/libonnxruntime.so* "${CAPI}"/libonnxruntime_providers_*.so "${DEST}/"
+echo "==> runtime in ${DEST}:"
+ls -1 "${DEST}" | sed 's/^/    /'
+echo "    (the app finds it on its next launch; Settings › About › Inference says what it chose)"