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/ui/active.rs

@@ -10,7 +10,7 @@ use crate::{
 pub struct ActiveData {
     pub id: WidgetId,
     /// Where its drawing goes, in its region node's coordinates.
-    pub extent: UiRegion,
+    pub placement: UiRegion,
     /// What a fraction declared or reported under this widget is a fraction
     /// of, as a length of the window.
     pub frame: UiVec2,
@@ -29,7 +29,7 @@ pub struct ActiveData {
     /// The box it was asked in, in the parent's region-node coordinates: the
     /// box its drawing was made in and the one its contract is about. Its
     /// drawing is placed elsewhere by re-expression, never by asking again.
-    pub part: UiRegion,
+    pub region: UiRegion,
     /// The measured answer and its dependencies. A hint-only dependency or
     /// a widget first encountered during placement has no measurement yet.
     pub answer: Option<(Size, LayoutHolds)>,
@@ -40,8 +40,8 @@ pub struct ActiveData {
     pub re_asked: bool,
     /// What the widget reported, in window-unit lengths.
     pub size: Size,
-    /// The window and extent reads that this drawing holds for, and the
-    /// frame and box it pinned.
+    /// The window and region reads that this drawing holds for, and the
+    /// frame and region it pinned.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -51,7 +51,7 @@ pub struct ActiveData {
     pub depth: usize,
     pub textures: Vec<TextureHandle>,
     /// Its primitives, each keeping the box it was written in -- in this
-    /// widget's extent coordinates, which is what a move recomposes from.
+    /// widget's placement coordinates, which is what a move recomposes from.
     pub primitives: Vec<RetainedPrimitive>,
     /// An owned mask holds one reference independently of its primitives.
     pub mask_region: Option<UiRegion>,
@@ -68,8 +68,8 @@ pub struct ActiveData {
     /// Its alignment when it was last drawn, which a change to the property
     /// is found against.
     pub own_align: RegionAlign,
-    /// The movable region whose coordinates `extent` uses when this widget
-    /// does not own a region node.
+    /// The movable region whose coordinates its placement is in when this
+    /// widget does not own a region node.
     pub parent_move: MoveIdx,
     /// The mask its drawing is clipped to: one it set itself, or the one it
     /// inherited from whoever drew it.