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/landmarks.rs

@@ -43,6 +43,13 @@ pub const POINTS: usize = 106;
 /// The crop's edge as a multiple of the detector box's longer edge.
 const CROP_SCALE: f32 = 1.5;
 
+/// The span of the frame, in long-edge units, the packed form covers: a
+/// quarter of the frame outside each edge.
+pub const PACKED_RANGE: (f32, f32) = (-0.25, 1.25);
+
+/// Bytes the packed form of one face's landmarks takes.
+pub const PACKED_BYTES: usize = POINTS * 4;
+
 /// Point indices of the subject's right eye's lid contour (image-left).
 pub const RIGHT_EYE: [usize; 10] = [33, 34, 35, 36, 37, 38, 39, 40, 41, 42];
 /// Point indices of the subject's left eye's lid contour (image-right).
@@ -55,6 +62,49 @@ pub struct Landmarks {
 }
 
 impl Landmarks {
+    /// Storage form: `106 × (x, y)` as little-endian **`u16` fixed point**
+    /// over the frame, 424 bytes.
+    ///
+    /// Each coordinate is normalised by `long_edge` like the five points the
+    /// catalog already keeps, then mapped over [`PACKED_RANGE`] — a quarter
+    /// of the frame either side of it, because a landmark on a face at the
+    /// edge does land outside the image — onto 0..65535. That is 0.14 source
+    /// pixels on a 6000-pixel frame. `f16` would be the same size and worse:
+    /// its three significant figures near 1.0 are six pixels at that scale,
+    /// and the eye contour this is kept for is drawn to the pixel.
+    pub fn to_packed_bytes(&self, long_edge: f32) -> Vec<u8> {
+        let (lo, hi) = PACKED_RANGE;
+        let pack = |v: f32| -> [u8; 2] {
+            let t = ((v / long_edge - lo) / (hi - lo)).clamp(0.0, 1.0);
+            ((t * 65535.0).round() as u16).to_le_bytes()
+        };
+        let mut out = Vec::with_capacity(POINTS * 4);
+        for &(x, y) in &self.points {
+            out.extend_from_slice(&pack(x));
+            out.extend_from_slice(&pack(y));
+        }
+        out
+    }
+
+    /// [`Self::to_packed_bytes`] read back, into source pixels of a frame
+    /// with this `long_edge`. `None` for a blob of the wrong length.
+    pub fn from_packed_bytes(bytes: &[u8], long_edge: f32) -> Option<Self> {
+        if bytes.len() != POINTS * 4 {
+            return None;
+        }
+        let (lo, hi) = PACKED_RANGE;
+        let unpack = |b: &[u8]| -> f32 {
+            let t = u16::from_le_bytes([b[0], b[1]]) as f32 / 65535.0;
+            (t * (hi - lo) + lo) * long_edge
+        };
+        let mut points = [(0.0_f32, 0.0_f32); POINTS];
+        for (i, p) in points.iter_mut().enumerate() {
+            let at = i * 4;
+            *p = (unpack(&bytes[at..at + 2]), unpack(&bytes[at + 2..at + 4]));
+        }
+        Some(Self { points })
+    }
+
     /// The lid contour of the subject's right eye.
     pub fn right_eye(&self) -> [(f32, f32); 10] {
         RIGHT_EYE.map(|i| self.points[i])
@@ -183,3 +233,29 @@ impl Landmarker {
         Ok(Some(Landmarks { points }))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Packed and unpacked, every point comes back within a fifth of a
+    /// source pixel on a 6000-pixel frame — including one outside the
+    /// image, which a face at the edge does produce.
+    #[test]
+    fn dense_landmarks_round_trip_through_their_packed_bytes() {
+        let mut points = [(0.0_f32, 0.0_f32); POINTS];
+        for (i, p) in points.iter_mut().enumerate() {
+            *p = (i as f32 * 37.3 - 200.0, 5900.0 - i as f32 * 11.1);
+        }
+        let lm = Landmarks { points };
+        let bytes = lm.to_packed_bytes(6000.0);
+        assert_eq!(bytes.len(), PACKED_BYTES);
+        assert_eq!(PACKED_BYTES, 424);
+        let back = Landmarks::from_packed_bytes(&bytes, 6000.0).unwrap();
+        for (a, b) in lm.points.iter().zip(back.points.iter()) {
+            assert!((a.0 - b.0).abs() < 0.2, "{} vs {}", a.0, b.0);
+            assert!((a.1 - b.1).abs() < 0.2, "{} vs {}", a.1, b.1);
+        }
+        assert!(Landmarks::from_packed_bytes(&bytes[..100], 6000.0).is_none());
+    }
+}