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.

core/dr-face/examples/scan_bench.rs

@@ -46,11 +46,13 @@ fn main() {
     );
     for n in sizes {
         let (embeddings, crop_px, images) = population(n);
+        let gallery = vec![true; n];
         let faces = Faces {
             embeddings: &embeddings,
             dim: EMBEDDING_DIM,
             crop_px: &crop_px,
             images: &images,
+            gallery: &gallery,
         };
 
         let start = std::time::Instant::now();