Index the library's faces, and group them into people

Wires dr-face to dr-catalog: a background sweep that reads the proxy the
grid already built, detects, aligns, embeds and stores, then a clustering
pass that turns those embeddings into suggested people.

Detection runs on the Large thumbnail tier and nowhere else. That is what
makes the feature affordable -- a browsed library has already paid for
its proxies, so face indexing adds no RAW decode that was not already
happening -- and it is why an image whose proxy is missing is skipped
rather than fetched: requesting one here would put face indexing on the
network path FR-CULL-8 keeps it off.

The sweep keeps no cursor. It asks the catalog what is missing, so it
resumes after process death with no repeated work beyond the in-flight
image, and cancelling is dropping the receiver.

recluster writes only the suggested half. Confirmed faces go in as
anchors and come back untouched, and a cluster of one stays nameless --
naming every stray face would fill the People view with noise the user
then has to dismiss.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-face/src/detect.rs

@@ -209,13 +209,13 @@ impl Detector {
                         // left/top/right/bottom in units of the stride.
                         let (cx, cy) = ((c * stride) as f32, (r * stride) as f32);
                         let b = &boxes[idx * 4..idx * 4 + 4];
-                        let (x0, y0) = lb.to_source(cx - b[0] * s, cy - b[1] * s);
-                        let (x1, y1) = lb.to_source(cx + b[2] * s, cy + b[3] * s);
+                        let (x0, y0) = lb.into_source(cx - b[0] * s, cy - b[1] * s);
+                        let (x1, y1) = lb.into_source(cx + b[2] * s, cy + b[3] * s);
 
                         let k = &kps[idx * 10..idx * 10 + 10];
                         let mut landmarks = [(0.0_f32, 0.0_f32); 5];
                         for (p, lm) in landmarks.iter_mut().enumerate() {
-                            *lm = lb.to_source(cx + k[p * 2] * s, cy + k[p * 2 + 1] * s);
+                            *lm = lb.into_source(cx + k[p * 2] * s, cy + k[p * 2 + 1] * s);
                         }
 
                         raw.push(Detection {
@@ -340,7 +340,7 @@ impl Letterbox {
     }
 
     /// Input-space point back to source pixels.
-    fn to_source(&self, x: f32, y: f32) -> (f32, f32) {
+    fn into_source(self, x: f32, y: f32) -> (f32, f32) {
         ((x - self.pad_x) / self.scale, (y - self.pad_y) / self.scale)
     }
 }
@@ -353,7 +353,7 @@ mod tests {
     fn letterbox_round_trips_a_point() {
         let lb = Letterbox::fit(1024.0, 683.0);
         for &(x, y) in &[(0.0_f32, 0.0_f32), (512.0, 341.0), (1023.0, 682.0)] {
-            let (bx, by) = lb.to_source(x * lb.scale + lb.pad_x, y * lb.scale + lb.pad_y);
+            let (bx, by) = lb.into_source(x * lb.scale + lb.pad_x, y * lb.scale + lb.pad_y);
             assert!((bx - x).abs() < 1e-2, "{bx} vs {x}");
             assert!((by - y).abs() < 1e-2, "{by} vs {y}");
         }