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>

ui/dr-ui/src/lib.rs

@@ -22,6 +22,7 @@
 mod activity;
 mod collections_ui;
 mod derived_sync;
+pub mod faces;
 mod develop;
 mod export;
 mod gradient;