Say region and placement, not extent

The split box was named `region` and `placement` on 2026-09-17; `frame`
came back as a length and survived, `extent` did not. It stayed as the
name for both halves, distinguished only by prose: `draw_at` bound the
caller's `part` to a parameter called `extent`, and `ActiveData` held two
`UiRegion`s that `draw_at` wrote `part: extent` from.

The box a parent asks a widget in is now the region, and where its
drawing ends up is its placement. `Painter`'s four holds accumulators
become the one `LayoutHolds` they were assembled into, which also drops
the name mapping between them.

The cold dump of 400 depth-5 trees is byte-identical across the change.

tests/cases/determinism.rs

@@ -28,7 +28,7 @@ impl Widget for BranchesOnMeasurement {
             .len(Axis::X);
         let px = painter.to_px(measured.apply_leftover(), Axis::X);
 
-        let below = Place::Within(Part::From(UiSpan::new(cut, painter.extent_len(Axis::Y))));
+        let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::All), below];
         match px > Px::from_f32(self.threshold) {
             true => painter.widget_at(&self.wide, [None; 2], place),