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.
iris-ai
5642f2010a