Withhold the empty groups that fill the People rail

`load_people` returned every live person, and on the reference library
that was 14,268 rows of which 11,739 held no faces at all — 85 per cent
of the rail naming nobody and able to do nothing.

They are not a mystery. A regrouping pass creates a person per cluster;
the next pass moves those faces elsewhere and leaves the person it
emptied behind. `faces::prune_empty_unnamed` exists for exactly this and
runs only at the end of a pass, so nothing clears what accumulates
between them, and the Identity screen never prunes at all.

Each one cost a `for_person` query and a built row on every reload. This
withholds precisely the set the prune already treats as disposable —
empty, unnamed, not set aside — and no more.

Filtered rather than deleted: a screen is being drawn, not a catalog
repaired. Nothing is lost, a sync cannot resurrect what was never
removed, and the prune stays the one place that decides these can go.

An empty group with a *name* still shows. That one is not debris but the
symptom of a real failure — a named person whose faces were regrouped out
from under them — and hiding it would take away the only way to merge
them back.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/gestures.md

@@ -34,7 +34,7 @@ A face is either the system's guess or the user's judgement, and the two are nev
 
 This is the point of having identified anybody. Without it the screen is a filing cabinet with no drawer handles.
 
-<sub>`ui/dr-ui/ui/identity.slint:552`</sub>
+<sub>`ui/dr-ui/ui/identity.slint:571`</sub>
 
 ### Change how faces are grouped
 
@@ -43,7 +43,7 @@ This is the point of having identified anybody. Without it the screen is a filin
 
 The right match confidence is a property of your library, not of the model. "What would this do?" answers for this library without writing anything; names, confirmations and the groups you have set aside are kept whatever the dials say.
 
-<sub>`ui/dr-ui/ui/identity.slint:589`</sub>
+<sub>`ui/dr-ui/ui/identity.slint:608`</sub>
 
 ## Library grid