Put the grouping dials where the regrouping is

The merge probability was `dr_face`'s constant and the smallest group was
a bare `< 2` in the clustering pass. Both were tuned on one library —
1,813 faces of one photographer's family — and the quantity they optimise
is a property of the population, not of the model. A household at close
family resemblance and two thousand strangers at a wedding want different
answers, and neither of them is the reference library. The doc comment
already conceded the point and pointed at `face_index --tune`; a
photographer does not have a terminal.

So they are `FaceSettings` now, saved per device beside the cache budgets
and edited from the People screen — beside the Regroup button that
applies them and the rail that shows what they did, because a value
changed three screens away from its effect is one nobody can tune.

Moving them is safe by construction, which is why nothing asks for
confirmation: a regroup writes only the suggested half, and
confirmations, names and ignores enter as anchors and come back
unchanged. The smallest-group rule is applied only to groups the system
invented — a group the user named or set aside survives it whatever its
size, because a display preference does not overrule a judgement.

**Withdrawal, without which the setting does nothing visible.** Raising
the smallest group stops the pass creating small groups; it does not
remove the ones a previous pass made, because those still hold their
suggestions, so they are not empty, so the prune leaves them. The pass
now releases every unanchored face it did not place before pruning.

And a dial you cannot see the effect of is not a dial. "What would this
do?" runs the same population through the clusterer without opening a
transaction and reports groups, faces grouped and largest group — one row
of `--tune`'s table, on the user's own library, on a worker thread. The
line leads with the group count because that is the number that says
which side of the right setting you are on: it climbs as fragments are
gathered into people and falls as separate people start being welded,
while the grouped-face count rises straight through both.

The preview parks its poll timer in a slot of its own. A preview and a
regroup are allowed to be in flight together, and sharing the sweep's
single slot would have the second to start drop the first's timer —
visible as a Regroup that finished on its worker and never said so.

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

ui/dr-ui/src/lib.rs

@@ -1011,6 +1011,19 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
     // faces are ticked for a split.
     let identity = std::rc::Rc::new(identity_ui::IdentityController::new());
 
+    // Settings: cache ceilings, export defaults and the face grouping dials, in
+    // their own config file.
+    //
+    // Wired independently of every view below. It reads no library and holds no
+    // session, so it has nothing to be sequenced against — which is the reason
+    // it is a page reachable from anywhere rather than a panel inside one view.
+    // Hoisted this far up because three separate things need the same record:
+    // the export action, the importer, and the People screen's grouping dials.
+    // A controller scoped to any one wiring block would be gone by the time the
+    // others were built, and two controllers each holding their own copy would
+    // each save over the other.
+    let settings = settings_ui::SettingsController::new();
+
     // Launch screen: shown when there is nothing to display — no local paths
     // and no configured library. A user who has already signed in and chosen
     // a folder goes straight to their images (FR-NC-1).
@@ -1098,6 +1111,7 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
                 identity.clone(),
                 library.catalog(),
                 activity.clone(),
+                settings.clone(),
                 move || {
                     let conn = lib_store.session()?;
                     dr_thumbs::ThumbStore::open(&library::thumbs_dir(&conn.account))
@@ -1170,16 +1184,6 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
         }
     }
 
-    // Settings: cache ceilings and export defaults, in their own config file.
-    //
-    // Wired independently of every view above. It reads no library and holds no
-    // session, so it has nothing to be sequenced against — which is the reason
-    // it is a page reachable from anywhere rather than a panel inside one view.
-    // Hoisted out of the block below: the export action needs the same record
-    // the settings page edits, and a controller scoped to the wiring block
-    // would be gone by the time that callback is built.
-    let settings = settings_ui::SettingsController::new();
-
     // TRACES: FR-CAT-10 | FR-CAT-11 | FR-NC-7a | FR-NC-7b
     // Import: a card into the library, and on to the server.
     //