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.

apps/darkroom-desktop/src/main.rs

@@ -83,10 +83,14 @@ fn main() -> anyhow::Result<()> {
 /// `DARKROOM_ORT_DIR` is for a developer pointing at a runtime that is not
 /// installed — the wheel's `capi` directory, say. Then beside the executable
 /// and in the package's private library directory, for a package that
-/// bundles its own; then the Flatpak prefix; then the system library
-/// directory, for a distribution that ships ONNX Runtime as a package of its
-/// own. A system copy whose GPU providers do not load is not a problem: the
-/// probe builds a real session before believing a provider.
+/// bundles its own; then the user's own `runtime/` beside the models, where
+/// `tools/fetch-desktop-runtime.sh` puts one; then the Flatpak prefix; then
+/// the system library directory, for a distribution that ships ONNX Runtime
+/// as a package of its own. The user's copy outranks the system's because
+/// the system's is the one most likely to be built without the GPU
+/// providers, or against the wrong cuDNN — and a system copy whose providers
+/// do not load is not a problem, only a slower app: the probe builds a real
+/// session before believing a provider.
 fn runtime_dirs() -> Vec<PathBuf> {
     let mut dirs = Vec::new();
     if let Some(dir) = std::env::var_os("DARKROOM_ORT_DIR") {
@@ -98,6 +102,7 @@ fn runtime_dirs() -> Vec<PathBuf> {
             dirs.push(bin.join("../lib/darkroom"));
         }
     }
+    dirs.push(dr_ui::inference::user_runtime_dir());
     #[cfg(target_os = "linux")]
     dirs.extend([
         PathBuf::from("/app/lib/darkroom"),