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/painter.rs

@@ -258,7 +258,7 @@ impl<'a> Painter<'a> {
                 parent_move: self.move_idx,
                 region_node,
                 mask: self.mask,
-                given_len,
+                given_region: local,
                 offer_len,
                 offer_placement,
                 px,