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.

ui/dr-ui/src/inference.rs

@@ -60,6 +60,18 @@ pub fn init(runtime_dirs: Vec<PathBuf>) {
     crate::memory::evict_at(crate::memory::Tier::Gpu, dr_inference_engine::release_all);
 }
 
+/// Where a person can put a runtime by hand: `runtime/` beside the models,
+/// searched before any system library. The system copy on the reference
+/// desktop is built without TensorRT and against the wrong cuDNN, and a
+/// working one is four files from the `onnxruntime-gpu` wheel; this is
+/// where they go, and `tools/fetch-desktop-runtime.sh` puts them there.
+pub fn user_runtime_dir() -> PathBuf {
+    crate::library::shared_face_models_dir()
+        .parent()
+        .map(|p| p.join("runtime"))
+        .unwrap_or_else(|| PathBuf::from("runtime"))
+}
+
 /// Which form the current backend loads `detector` in, given the files on
 /// this device — the fact `faces.model_id` has to carry (§7).
 ///
@@ -94,8 +106,18 @@ pub fn about_lines() -> (String, String) {
             status.engines.0,
             status.engines.1
         )
-    } else {
+    } else if status.failed.is_empty() {
         status.reason
+    } else {
+        // Every rung that was tried and why it lost, not only the first:
+        // "TensorRT: not enabled in this build" says nothing about why CUDA
+        // was not taken instead.
+        status
+            .failed
+            .iter()
+            .map(|(rung, why)| format!("{}: {why}", rung.label()))
+            .collect::<Vec<_>>()
+            .join(" · ")
     };
     (line, detail)
 }