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/Cargo.toml

@@ -24,6 +24,9 @@ ndarray = { workspace = true, optional = true }
 # tree for embedded previews.
 zune-jpeg.workspace = true
 env_logger.workspace = true
+# The probe example drives `ort` directly to print the raw load error, and
+# asks the engine for a runtime by name rather than naming one itself.
+dr-inference-engine.workspace = true
 
 [features]
 # On by default: a local adjustment that cannot select a subject is half the