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.

examples/text.rs

@@ -26,6 +26,7 @@ impl DefaultAppState for State {
             .wrap(true)
             .text_align(Align::LEFT)
             .pad(16)
+            .width(rel(1.0))
             .background(panel());
 
         // Each one takes the whole width, because `text_align` puts the