Keep each face's quality, and never compare against a poor one

The embedder's raw output has a length, and the length is a reading of
how recognisable the crop was: a blur, an occlusion or a hard profile
comes out short. Normalising threw it away. A short vector sits near
the middle of the sphere and matches a little of everyone, which is how
one bad crop bridges two people in a grouping pass.

So the length is kept — the store now holds the raw vector, re-normalised
on load, with the length beside it as `faces.quality` — and a face under
MIN_GALLERY_QUALITY (14) is a probe: measured against the gallery and
placed where it fits, but never what another face is measured against.
Two probes are never paired, and a probe is nobody's evidence for a
confidence. The People screen shows the number as "Quality 17.3", dimmed
below the floor.

Faces indexed before this stored unit vectors and have no reading; they
are admitted to the gallery, and schema V14 forgets the run marker of
every image holding one so the next indexing pass measures them. A
peer's unmeasured shard faces are not adopted, or a sync would write
that marker back.

ui/dr-ui/ui/identity.slint

@@ -51,6 +51,15 @@ export struct IdentityFace {
     // with has causes, and "the face was 41 pixels across" is one the user can
     // act on by finding a better photograph.
     crop-px: int,
+    // "Quality 17.3" — the model's own reading of how recognisable the crop
+    // was, composed in Rust like `confidence` and for the same reason. The
+    // other cause a bad suggestion can have, and the one the grouping pass
+    // acts on: a face below the floor is placed but never compared against.
+    quality: string,
+    // Whether the face clears that floor. Drawn, not just known, because a
+    // group that has gathered none of a person's other photographs has an
+    // explanation the user can only see if every member's label says it.
+    in-gallery: bool,
     // Part of the current multi-select — what a split would carry.
     picked: bool,
 }
@@ -110,6 +119,28 @@ component FaceCell inherits Rectangle {
                 vertical-alignment: center;
             }
 
+            // The quality, over the foot of the crop. In the picture rather
+            // than the caption because the caption is the verdict controls'
+            // and at the compact width there is no room beside them; and
+            // dimmed below the floor, which is the one state of it that
+            // changes what the grouping does.
+            Rectangle {
+                x: 0;
+                y: parent.height - self.height;
+                width: parent.width;
+                height: 16px;
+                background: Theme.ground.with-alpha(0.55);
+                Text {
+                    text: face.quality;
+                    color: face.in-gallery ? Theme.ink-dim : Theme.warn-ink;
+                    font-size: Theme.text-sm;
+                    horizontal-alignment: center;
+                    vertical-alignment: center;
+                    width: parent.width;
+                    height: parent.height;
+                }
+            }
+
             // A confirmed face carries a quiet marker rather than a badge: the
             // grid is mostly confirmed once the user has worked through it, and
             // a loud mark on the common case is just noise.