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/src/lib.rs

@@ -75,8 +75,10 @@ pub use cluster::{
 #[cfg(feature = "inference")]
 pub use detect::{DetectOptions, Detection, Detector};
 #[cfg(feature = "inference")]
-pub use embed::Embedder;
-pub use embedding::{Embedding, ModelId, EMBEDDING_DIM};
+pub use embed::{Embedded, Embedder};
+pub use embedding::{
+    in_gallery, read_f16_bytes, Embedding, ModelId, EMBEDDING_DIM, MIN_GALLERY_QUALITY,
+};
 pub use naming::{name_for_instance, name_instances, NamedFace};
 
 /// What can go wrong between an image and a face.