Add dr-inference-engine and route every model session through it

One crate names the runtime, the providers and the devices; dr-face and
dr-segment ask it for a session by role. It hands ort an API table once
per process — from a libonnxruntime it dlopens when the app names a
directory holding one, otherwise from tract — so the Rust build stays
free of C on every target and a package can install the runtime as a
file (docs/inference.md §3).

Sessions live in a registry behind a Model handle that holds the bytes,
not the session: every use refreshes a timestamp and a reaper unloads
whatever sat idle past the decay. A scan that runs the detector on each
image never lets it go idle; a click in the develop view lets the
segmenter go after thirty seconds; a handle used after that reloads,
and reloads on a higher rung if a compiled engine has landed meanwhile.

The probe walks the platform's ladder by building strict sessions and
timing them against the CPU provider, caches the choice against a
fingerprint of the runtime, driver, hardware and models, and compiles
engines for the selected rung in the background, smallest model first.
Nothing in this commit turns the native path on: the apps still run on
tract until they call init with a runtime directory.

core/dr-segment/src/lib.rs

@@ -98,3 +98,13 @@ pub enum SegmentError {
     #[error("category descriptor: {0}")]
     CategoryDescriptor(String),
 }
+
+#[cfg(feature = "semantic")]
+impl From<dr_inference_engine::Error> for SegmentError {
+    fn from(e: dr_inference_engine::Error) -> Self {
+        match e {
+            dr_inference_engine::Error::Inference(e) => SegmentError::Inference(e),
+            dr_inference_engine::Error::Io(e) => SegmentError::ModelRead(e),
+        }
+    }
+}