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/Cargo.toml

@@ -11,10 +11,11 @@ build = "build.rs"
 thiserror.workspace = true
 log.workspace = true
 
-# Inference. `ort` is the API; **tract is the engine** — see the workspace
-# manifest for why the C++ ONNX Runtime is not linked here.
+# Inference. `ort` is the API; **what runs it is `dr-inference-engine`'s
+# business** — tract, or an ONNX Runtime the app found on disk, on whichever
+# provider the device has (docs/inference.md). This crate never names either.
 ort = { workspace = true, optional = true }
-ort-tract = { workspace = true, optional = true }
+dr-inference-engine = { workspace = true, optional = true }
 ndarray = { workspace = true, optional = true }
 
 [dev-dependencies]
@@ -35,7 +36,7 @@ default = ["semantic", "embedded-model"]
 # Separable because the watershed half is genuinely independent of it: with
 # this off, `dr-segment` is a pure-CPU graph algorithm crate with no model to
 # carry, which is what the headless hierarchy tests want.
-semantic = ["dep:ort", "dep:ort-tract", "dep:ndarray"]
+semantic = ["dep:ort", "dep:dr-inference-engine", "dep:ndarray"]
 
 # Compile the weights into the binary.
 #