Resolve a declared length where the widget is drawn, not inside it

SetSize took its declared length out of UiRegion::FULL, which is the box
it was already given, so a span that had sized that box from the same
hint had the fraction taken twice: .width(rel(0.5)) in a 400-wide span
drew its child 100 wide. It held under a Pad or the root, which do not
honour a hint, and hid under px, where 200 of a 200-wide box is all of
it. The text example was 111,923 pixels from upstream/main because of
it.

Whoever draws a widget now takes its declared length, in its own box,
which is what a fraction of one means, and is the identity for a caller
that already reserved the space. rest is not taken: a share of what is
left over is only a length to the widget dividing one, so it passes up
in the size as it does out of a span. SetSize keeps only what it
declares.

A declared length is then part of the box its parent decided, so
changing one has to redraw the parent; the lengths resolved into a box
are kept beside it and compared. Assuming instead that any dirty widget
which declares a length needs its parent costs 17% of a frame that
dirties 130 of 260 widgets, and buys nothing.

All five reference renders, the resize render and the image replay are
byte-identical to upstream/main, the 100-seed sweep passes, and the
resize fixture is 1.286 ms against 1.289 before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/ui/active.rs

@@ -1,5 +1,6 @@
 use crate::{
-    LayerId, MaskIdx, MoveIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId, util::Vec2,
+    LayerId, Len, MaskIdx, MoveIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId,
+    util::Vec2,
 };
 
 /// important non rendering data for retained drawing
@@ -38,6 +39,10 @@ pub struct ActiveData {
     /// The slot its primitives are positioned through: its own if its parent
     /// placed it, otherwise the nearest ancestor that has one.
     pub move_idx: MoveIdx,
+    /// The declared lengths whoever drew this widget resolved into its box.
+    /// A change to one moves a box this widget cannot fix by drawing again,
+    /// and comparing them is what says so.
+    pub declared: [Option<Len>; 2],
     /// The slot `region` is given in, which is whatever its parent drew in.
     pub parent_move: MoveIdx,
     pub mask: MaskIdx,