WIP: a widget's region stays put and its placement moves in it

The protocol split: `region` is the box a parent gives a widget -- what a
fraction it declares or reports is a fraction of, and the coordinates
every region it writes composes within -- and it is the same box on the
ask that measures and the ask that places. `placement` is what of that
region the drawing takes, chosen by the parent per axis or by the
widget's own answer and alignment.

That is what stops a fraction being resolved twice: the placing ask no
longer hands the widget its own answer as its box, so nothing under it
re-resolves against a box that came from its own report. `reports_of`
and `decided` are gone, folded into the two regions; `box_of` is gone;
`declared_box` becomes `ask_box`, which gives a rule the region's length
and takes the position from the placement.

84 of 92 suite tests pass. Five text and region-node cases still diverge
warm against cold, and three count a second widget draw where a span's
measuring ask and its placing ask give different placements.

tests/cases/layout.rs

@@ -20,11 +20,12 @@ fn a_span_gives_each_child_the_width_it_asked_for() {
     assert_corners!(h, right, (100, 0), (400, 200));
 }
 
-/// A span offers each child the room left after the one before, because a
-/// text has to wrap at the width actually there, but reads what the child
-/// reports as a fraction of the whole row. So two children asking for half
-/// each take the whole row between them, however much of it was left when
-/// each was asked, and a third overflows.
+/// A span places each child in the room left after the one before, because a
+/// text has to wrap at the width actually there, but the child's region is
+/// the whole row. So two children asking for half each take the whole row
+/// between them, however much of it was left when each was asked, and a third
+/// overflows -- and a span passes its own region on unchanged, so a child of
+/// a nested span asking for half asks for half of the same row.
 #[test]
 fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
     let mut h = Harness::new((400, 100));
@@ -34,10 +35,10 @@ fn a_span_reads_a_child_report_as_a_fraction_of_the_row() {
     let tail = rect(Color::BLUE).width(100).add(&mut h.rsc);
     h.set_root((half, nested, tail).span(Dir::RIGHT).width(rel(1.0)));
 
-    // The nested span is placed at the length it reported and drawn there
-    // once more; half of that final box is what its own child takes.
+    // The nested span is placed at the length it reported, and its own child
+    // asks for half of the row rather than half of that placement.
     assert_corners!(h, nested, (200, 0), (400, 100));
-    assert_corners!(h, inner, (200, 0), (300, 100));
+    assert_corners!(h, inner, (200, 0), (400, 100));
     assert_corners!(h, tail, (400, 0), (500, 100));
 }