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.

Cargo.lock

@@ -1475,11 +1475,11 @@ dependencies = [
 name = "dr-face"
 version = "0.12.2"
 dependencies = [
+ "dr-inference-engine",
  "env_logger",
  "log",
  "ndarray",
  "ort",
- "ort-tract",
  "thiserror 2.0.20",
  "zune-jpeg 0.4.21",
 ]
@@ -1511,6 +1511,20 @@ dependencies = [
  "wgpu",
 ]
 
+[[package]]
+name = "dr-inference-engine"
+version = "0.12.2"
+dependencies = [
+ "libloading",
+ "log",
+ "ort",
+ "ort-sys",
+ "ort-tract",
+ "serde",
+ "serde_json",
+ "thiserror 2.0.20",
+]
+
 [[package]]
 name = "dr-ingest"
 version = "0.12.2"
@@ -1584,11 +1598,11 @@ dependencies = [
 name = "dr-segment"
 version = "0.12.2"
 dependencies = [
+ "dr-inference-engine",
  "env_logger",
  "log",
  "ndarray",
  "ort",
- "ort-tract",
  "thiserror 2.0.20",
  "zune-jpeg 0.4.21",
 ]