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/scene.rs

@@ -59,7 +59,7 @@ use ndarray::ArrayView3;
 
 #[cfg(test)]
 use crate::semantic::INPUT_EDGE;
-use crate::semantic::{install_backend, Letterbox, Window};
+use crate::semantic::{Letterbox, Window};
 use crate::SegmentError;
 
 /// Classes in the ADE20K vocabulary the scene model was trained on.
@@ -84,7 +84,7 @@ pub struct Category {
 
 /// The scene model, and the categories it has been told to report.
 pub struct SceneModel {
-    session: ort::session::Session,
+    session: dr_inference_engine::Model,
     categories: Vec<Category>,
 }
 
@@ -132,12 +132,12 @@ impl SceneModel {
     }
 
     pub fn from_bytes(bytes: &[u8], categories: Vec<Category>) -> Result<Self, SegmentError> {
-        install_backend();
-
-        let session = ort::session::Session::builder()
-            .map_err(SegmentError::Inference)?
-            .commit_from_memory(bytes)
-            .map_err(SegmentError::Inference)?;
+        // f32, as for `SemanticModel`; see there.
+        let session = dr_inference_engine::open(
+            dr_inference_engine::Role::Scene,
+            dr_inference_engine::Form::F32,
+            bytes,
+        )?;
 
         Ok(Self {
             session,
@@ -170,13 +170,9 @@ impl SceneModel {
             });
         }
 
-        // Split the borrow: `run` needs the session mutably while
-        // `marginalise` needs the categories, and going through `self` for
-        // both at once is what the borrow checker objects to.
-        let Self {
-            session,
-            categories,
-        } = self;
+        let categories = &self.categories;
+        let acquired = self.session.acquire()?;
+        let mut session = acquired.lock();
 
         let window = Window {
             x: 0.0,