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.

core/src/layout_diagnostics.rs

@@ -54,11 +54,11 @@ pub(crate) enum Counter {
     GlyphPlacements,
     OutsidePinnedLen,
     OutsideFrame,
-    OutsideExtent,
+    OutsideRegion,
 }
 
 impl Counter {
-    const COUNT: usize = Self::OutsideExtent as usize + 1;
+    const COUNT: usize = Self::OutsideRegion as usize + 1;
 
     const NAMES: [&'static str; Self::COUNT] = [
         "updates",
@@ -90,7 +90,7 @@ impl Counter {
         "glyph placements",
         "reuse outside: the length it was pinned to",
         "reuse outside: a frame length",
-        "reuse outside: an extent length",
+        "reuse outside: a region length",
     ];
 }