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.

core/src/ui/render_state.rs

@@ -1,6 +1,6 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
-use crate::ui::painter::declared_len;
+use crate::ui::painter::declared_lens;
 use crate::{
     ActiveData, Axis, DrawLayers, Holds, IdLike, Len, MaskIdx, MoveIdx, Moves, Painter,
     PixelRegion, Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, WidgetId, Widgets,
@@ -256,6 +256,14 @@ impl UiRenderState {
             "'{}' ({id:?}) drew a size its size_hint disagrees with",
             rsc.widgets().label(id)
         );
+        // A rule wins on the axis it names, and the draw answers the rest.
+        // Applied here so it is one place rather than every widget that could
+        // carry one, and so the widget under a rule never learns of it.
+        let rules = rsc.widgets().size_rules(id);
+        let size = Size {
+            x: rules.x.apply(size.x),
+            y: rules.y.apply(size.y),
+        };
         let holds = [own[0].and(under[0]), own[1].and(under[1])];
         debug_assert!(
             holds[0].contains(px.x) && holds[1].contains(px.y),
@@ -739,11 +747,7 @@ impl UiRenderState {
         // whether to draw it at all, so a change to either is the parent's
         // to draw -- with the mark left on, so the parent draws it rather
         // than keeping it.
-        let declared_changed = rsc.widgets().get_dyn(id).is_some_and(|widget| {
-            AXES.into_iter()
-                .zip(active.declared)
-                .any(|(axis, was)| declared_len(widget, axis) != was)
-        });
+        let declared_changed = declared_lens(rsc.widgets(), id) != active.declared;
         if let Some(parent) = active.parent
             && (declared_changed || !active.drawn)
         {