Keep the dense landmarks behind each eye reading, packed

The 106 points the eye boxes were cut from, stored beside the reading as
16-bit fixed point over the frame: 424 bytes a face, a seventh of a pixel
on a 6000-pixel frame, where f16 at the same size would have been six.
Derived data like the embedding, kept for the same reason — it cost a
fetch and a model run, and the next per-face pass should run from the
catalog. Shards carry it; a peer's shard from before it is still read.

core/dr-face/src/classify.rs

@@ -39,7 +39,7 @@ use crate::align::{
     SUNGLASSES_EDGE,
 };
 use crate::eyes::{Eye, EyeReading};
-use crate::landmarks::Landmarker;
+use crate::landmarks::{Landmarker, Landmarks};
 use crate::{install_backend, FaceError, Pixels};
 
 /// A loaded OCEC graph.
@@ -202,13 +202,18 @@ impl EyeModels {
         })
     }
 
-    /// Read one face's eyes.
+    /// Read one face's eyes, and hand back the dense landmarks it read them
+    /// from.
     ///
     /// `bbox` is the detector's `(x0, y0, x1, y1)` and `landmarks5` its five
     /// points, both in source pixels; the buffer is the one the aligned
     /// crop was taken from, so an eye is read from the same pixels the
     /// embedder saw the face in. `None` where nothing could be cut — a
     /// degenerate box or landmarks — which the caller stores as "not read".
+    ///
+    /// The landmarks come back because they cost a model run the caller will
+    /// not want to pay twice: stored beside the reading, a later pass over
+    /// faces — head pose, expression — has them without the original.
     pub fn read(
         &mut self,
         px: Pixels<'_>,
@@ -216,7 +221,7 @@ impl EyeModels {
         height: usize,
         bbox: (f32, f32, f32, f32),
         landmarks5: &[(f32, f32); 5],
-    ) -> Result<Option<EyeReading>, FaceError> {
+    ) -> Result<Option<(EyeReading, Landmarks)>, FaceError> {
         let Some(lm) = self.landmarks.landmarks(px, width, height, bbox)? else {
             return Ok(None);
         };
@@ -249,10 +254,11 @@ impl EyeModels {
         };
         let right = eye(&lm.right_eye())?;
         let left = eye(&lm.left_eye())?;
-        Ok(Some(EyeReading {
+        let reading = EyeReading {
             right,
             left,
             sunglasses: self.sunglasses.classify(&head)?,
-        }))
+        };
+        Ok(Some((reading, lm)))
     }
 }