WIP: a stack does not take its sizing child's fraction twice

Every child gets the whole of the stack's box rather than `box_of(size)`,
and `widget_at` does not resolve a rule into a box already chosen from it.
Fixes half a row becoming a quarter, which no oracle can see because warm
and cold shrink alike. Pinned by
`a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice`.

Not landed. Seed 1091 at depth 4, `shuffle-swap-for-three`, disagrees by
three steps of the grid -- warm 1053.9971 against cold 1054 -- where the
oracle tolerates two. Not a structural divergence: `box_of` was also making
a child's placement exact, by handing it a box of exactly the length it
asked for, and giving it the whole box instead puts a rounding back at each
nesting level. Two nested stacks is three steps. Widening AGREE_STEPS is
not the answer; finding the composition that went from exact to rounded is.

Everything else is green: suite, shrinker at 400 seeds of depth 5, oracle at
1000 seeds of depth 6, and the rest of 2000 seeds at depth 4.

Also carries examples/text.rs's top panel taking the full width.

core/src/ui/painter.rs

@@ -168,10 +168,20 @@ impl<'a> Painter<'a> {
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
         let align = self.rsc.widgets().alignment(id.id());
+        // A rule this box was already chosen from is not resolved into it a
+        // second time. The box is that rule's length already, so resolving
+        // it again takes the fraction twice -- a widget declaring half of a
+        // stack, in the stack its own answer made half a row, is a quarter
+        // of the row. Pixels survive it, being the same length wherever they
+        // are taken from, which is why only a share ever shrank.
+        let resolve = AXES.map(|axis| match decided[axis as usize] {
+            true => None,
+            false => declared[axis as usize],
+        });
         // Composing `FULL` through a box is not quite the identity in f32,
         // so a child with nothing declared keeps the box it would have had.
-        let local = match declared.iter().any(Option::is_some) {
-            true => declared_box(region, declared, align),
+        let local = match resolve.iter().any(Option::is_some) {
+            true => declared_box(region, resolve, align),
             false => region,
         };
         let within = match local == UiRegion::FULL {
@@ -393,14 +403,6 @@ impl<'a> Painter<'a> {
             .is_some()
     }
 
-    /// The part of this widget's box that something of `size` takes, at the
-    /// near edge. A container that reports one child's size gives every child
-    /// this, so what it draws is inside what it says it occupies.
-    pub fn box_of(&self, size: Size) -> UiRegion {
-        let lens = placed_lens(size, [None; 2], [false; 2]);
-        placed_box(UiRegion::FULL, lens, RegionAlign::NEAR)
-    }
-
     /// This widget's box in pixels. Reading it makes the drawing one that
     /// holds for this box only, until `holds` says how far it goes.
     pub fn px_size(&mut self) -> PxVec2 {