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/layout.rs

@@ -124,7 +124,7 @@ fn padding_keeps_the_frame_distinct_from_the_room_left_in_a_row() {
     h.set_root((icon, padded).span(Dir::RIGHT).width(rel(1.0)));
     let active = &h.render.active[&text.id()];
     let window = h.render.output_size().x;
-    let asked = active.part.x.len().to_px(window);
+    let asked = active.region.x.len().to_px(window);
     assert_eq!(active.frame.x.to_px(window), Px::from_int(868));
     assert_eq!(asked, Px::from_int(844));
 }
@@ -171,7 +171,7 @@ fn padding_narrows_both_frame_and_box_inside_a_share() {
     let active = &h.render.active[&text.id()];
     let window = h.render.output_size().x;
     assert_eq!(active.frame.x.to_px(window), Px::from_int(418));
-    assert_eq!(active.part.x.len().to_px(window), Px::from_int(418));
+    assert_eq!(active.region.x.len().to_px(window), Px::from_int(418));
 }
 
 #[test]
@@ -500,7 +500,7 @@ fn a_row_of_equal_shares_fills_it_exactly() {
 /// a step of. Kept in step with `snap_floor` in `prelude.wgsl`.
 fn drawn_edges(h: &Harness, id: WidgetId, axis: Axis) -> (f32, f32) {
     let active = &h.render.active[&id];
-    let region = h.render.moves.resolve(active.move_idx, active.extent);
+    let region = h.render.moves.resolve(active.move_idx, active.placement);
     let dim = h.size().axis(axis);
     let snap = |v: f32| (v + Px::STEP.to_f32() * 0.5).floor();
     let edge = |s: Len| snap(s.rel.to_f32() * dim + s.px.to_f32());