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.

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

@@ -115,17 +115,24 @@ fn finish(cache: Cache) {
     s.probing = false;
 }
 
-/// The smallest configured model: the detector on every device shipped
-/// today, and a ~2 MB graph is the cheapest real test of a provider.
+/// The smallest detector, or the smallest model of any role if there is
+/// none. A ~2 MB detector is the cheapest real test of a provider, and the
+/// detector is the role the int8 forms exist for — the eye classifiers are
+/// smaller still, and a Hexagon probed with one would fail for want of a
+/// form nobody ships.
 fn probe_model(cfg: &Config) -> Option<(Role, PathBuf)> {
-    cfg.models
-        .iter()
-        .filter_map(|(role, path)| {
-            let size = std::fs::metadata(path).ok()?.len();
-            Some((size, *role, path.clone()))
-        })
-        .min_by_key(|(size, _, _)| *size)
-        .map(|(_, role, path)| (role, path))
+    let smallest = |want: Option<Role>| {
+        cfg.models
+            .iter()
+            .filter(|(role, _)| want.is_none_or(|w| *role == w))
+            .filter_map(|(role, path)| {
+                let size = std::fs::metadata(path).ok()?.len();
+                Some((size, *role, path.clone()))
+            })
+            .min_by_key(|(size, _, _)| *size)
+            .map(|(_, role, path)| (role, path))
+    };
+    smallest(Some(Role::Detector)).or_else(|| smallest(None))
 }
 
 /// Build strictly, run once for the engine, then time three runs; the