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/span.rs

@@ -13,7 +13,7 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
-        let far = painter.extent_len(axis);
+        let far = painter.region_len(axis);
         let along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),