Carry a length as a rule beside a widget, not a widget around it

`.width()` built a `SetSize` whose whole job was to answer `size_hint`, so
every declared length cost a widget, an `ActiveData` and a link of chain to
say one number. It is now a `SizeRule` per axis on `WidgetData`, beside
`region_node`, resolved by `Painter` where the widget is drawn. `SetSize`
and `MaxSize` are gone; `MaxSize` had no caller but its own builders.

That settles which of two answers is the size. A rule wins on the axis it
names and the `Size` returned by `draw` answers the rest, applied once in
`draw_inner` rather than by each widget that could carry one -- so the
widget under a rule never learns of it. `Painter::size_hint` reads the rule
first for the same reason: a rule that beats what a widget would draw has
to beat what it says about itself.

`declared_lens` still falls back to a non-leftover `size_hint`, which is
how an image or a gap gets its own pixel size rather than the whole offer.
That is the offer's business rather than a declaration's, and it falls away
when a widget occupies its reported size inside the box it was offered.

`known` and `declared` are separate because a share is a length to whoever
divides one and not to whoever composes a box: `.width(leftover(3))` is
known without drawing but cannot narrow anything.

Checked: fmt, clippy, 85 tests, and 100 generated seeds agreeing warm
against cold in 67.6 s. `minimal`, `text` and `view` render byte-identical
at 1920x1200; `tabs` differs only in the widget count it prints about
itself, which is two wrapper types smaller.

tests/retained.rs

@@ -236,7 +236,7 @@ fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
     h.set_root(parent);
     assert_corners!(h, inner, (0, 0), (400, 80));
 
-    h.rsc[inner].y = Some(Len::px(120));
+    h.set_len(inner, Axis::Y, 120);
     h.frame();
 
     assert_corners!(h, inner, (0, 0), (400, 120));
@@ -484,7 +484,7 @@ fn stretching_a_subtree_carries_the_children_in_it() {
     let settled = draws.get();
     assert_corners!(h, inner, (0, 40), (400, 400));
 
-    h.rsc[first].y = Some(Len::px(80));
+    h.set_len(first, Axis::Y, 80);
     h.frame();
 
     assert_eq!(
@@ -508,7 +508,7 @@ fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
     h.set_root((bar, row).span(Dir::RIGHT));
     let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     // The span reads every child's size, so redrawing one takes the span
@@ -534,7 +534,7 @@ fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
     h.set_root((bar, row).span(Dir::RIGHT));
     let settled = draws.get();
 
-    h.rsc[bar].x = Some(Len::px(200));
+    h.set_len(bar, Axis::X, 200);
     h.frame();
 
     assert_eq!(draws.get(), settled, "its own length did not change");