Keep the embedder off the Hexagon, and let the probe example ask for a runtime

On the tablet the engine compiled arcface for the NPU: the routing
compared the form a rung wants with the form on offer, and for the
embedder both are f32, so nothing said no. A rung now says which roles
it serves at all, and the Hexagon does not serve the embedder (§7 —
its vectors must compare across devices). Tested at the routing seam.

dr-segment's onnx_probe example still named ort-tract, which is what
stopped the workspace test build.

core/dr-segment/examples/onnx_probe.rs

@@ -34,7 +34,7 @@ fn main() {
     let bytes = std::fs::read(&path).expect("read model");
     println!("{path}: {} bytes", bytes.len());
 
-    let _ = ort::set_api(ort_tract::api());
+    dr_inference_engine::ensure_runtime();
     let t = Instant::now();
     let mut session =
         match ort::session::Session::builder().and_then(|mut b| b.commit_from_memory(&bytes)) {