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.

src/widget/position/offset.rs

@@ -12,8 +12,8 @@ impl Widget for Offset {
         // without the offset.
         let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
         let place = [
-            moved(painter.extent_len(Axis::X), self.amt.x),
-            moved(painter.extent_len(Axis::Y), self.amt.y),
+            moved(painter.region_len(Axis::X), self.amt.x),
+            moved(painter.region_len(Axis::Y), self.amt.y),
         ];
         painter.widget_at(&self.inner, [None; 2], place).size()
     }