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/src/library.rs

@@ -5937,6 +5937,7 @@ mod tests {
             confidence: 0.9,
             embedding: vec![0u8; 1024],
             crop_px: 120.0,
+            quality: None,
             crop: Vec::new(),
             model_id: "w600k_mbf".into(),
         };
@@ -5986,6 +5987,7 @@ mod tests {
             confidence: 0.9,
             embedding: vec![0u8; 1024],
             crop_px: 120.0,
+            quality: None,
             crop: Vec::new(),
             model_id: "w600k_mbf".into(),
         };
@@ -7012,6 +7014,7 @@ mod tests {
                 confidence: 0.9,
                 embedding: vec![0u8; 1024],
                 crop_px: 120.0,
+                quality: None,
                 crop: Vec::new(),
                 model_id: "w600k_mbf".into(),
             };