Recompose retained frames exactly and preserve text width validity

Keep each widget's original local frame and replay the same composition
order on reuse. Remove inverse region remapping, including its fixed-frame
fallback that forced otherwise valid subtrees to draw again.

Require exact pixel-region equality in the shared generated oracle. Check
primitive and mask geometry as well as draw reuse when fixed frames resize.
Publish text's retained line-break range, with no upper bound when there
are no soft breaks, and cover widening, explicit newlines, and empty text.

Compared with efb416b, the depth-8 diagnostic rig performs 7-9% fewer widget
evaluations in the affected phases. Uninstrumented release runs use 3.5%
fewer instructions for size changes and 5.0% fewer for resize. Repaint and
scroll use 0.7% and 0.6% more instructions. Container updates remain substantially more expensive than the e44dea3 baseline;
this is still an experimental continuation, not a production replacement.

core/src/ui/active.rs

@@ -15,10 +15,9 @@ pub struct ActiveData {
     pub region: UiRegion,
     /// Where its drawing sits inside that box, in the box's own coordinates.
     pub placement: UiRegion,
-    /// The same box as lengths of its parent's box, which is the one route
-    /// to a box in pixels: a draw threads these down a level at a time, and
-    /// [`crate::UiRenderState::redraw`] takes the same steps back up.
-    pub given_len: UiVec2,
+    /// The original frame in its parent widget's coordinates. Recomposition
+    /// and pixel-length evaluation both follow this chain.
+    pub given_region: UiRegion,
     /// The lengths of the box its parent first asked about it in, as
     /// lengths of the box the parent was itself offered. Any later box it
     /// was given was decided knowing its answer, so this is the question