Move the panorama keypoint detector onto the engine, and probe with a detector

XFeat's two exports are a Keypoints role now; the crate no longer names
tract, and the app compiles TensorRT engines for both ahead of the
first merge. The probe picks the smallest *detector* rather than the
smallest file: the tablet's first run chose the 112 KB eye classifier,
which has no int8 form, and reported the Hexagon as failed for want of
one.

core/dr-inference-engine/src/lib.rs

@@ -39,6 +39,8 @@ pub enum Role {
     Landmarks,
     /// The eye-state and sunglasses classifiers, a few hundred kilobytes.
     EyeClassifier,
+    /// XFeat, the panorama keypoint detector (docs/panorama.md).
+    Keypoints,
 }
 
 /// Which numeric form of a model a session was built from.