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/painter.rs

@@ -2,7 +2,7 @@
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
     Axis, Holds, Len, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Widget, WidgetId,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
         PrimitiveKind, TexturePrimitive,
@@ -111,16 +111,12 @@ impl<'a> Painter<'a> {
         self.widget_at(id, region)
     }
 
-    /// What a widget declares its lengths to be, which whoever draws it
-    /// resolves into its box. `leftover` is not among them: a part of what is
-    /// left over is only a length to the widget dividing one, so it passes
-    /// up in the size instead. Reading it depends on nothing -- the box that
-    /// comes of it is kept on the child, and `redraw` compares it there.
+    /// What a widget's rules declare its lengths to be, which whoever draws
+    /// it resolves into its box. Reading them depends on nothing -- the box
+    /// that comes of them is kept on the child, and `redraw` compares it
+    /// there.
     fn declared_lens<W: ?Sized>(&self, id: &StrongWidget<W>) -> [Option<Len>; 2] {
-        let Some(widget) = self.rsc.widgets().get_dyn(id.id()) else {
-            return [None; 2];
-        };
-        AXES.map(|axis| declared_len(widget, axis))
+        declared_lens(self.rsc.widgets(), id.id())
     }
 
     /// Takes back a child that was drawn only to find out how long it is.
@@ -201,11 +197,14 @@ impl<'a> Painter<'a> {
     /// What a child says its length is without being drawn, if it can say.
     /// Asking counts as reading its size.
     pub fn size_hint<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
-        let hint = self
-            .rsc
-            .widgets()
-            .get_dyn(id.id())
-            .and_then(|widget| widget.size_hint(axis));
+        let widgets = self.rsc.widgets();
+        // A rule is the answer where there is one: it wins over whatever the
+        // widget would draw, so it has to win over what the widget says too.
+        let hint = widgets.size_rules(id.id()).axis(axis).known().or_else(|| {
+            widgets
+                .get_dyn(id.id())
+                .and_then(|widget| widget.size_hint(axis))
+        });
         #[cfg(feature = "layout-diagnostics")]
         diag::hint_read(id.id(), self.id, axis, hint);
         match hint {
@@ -439,8 +438,21 @@ impl PrimitiveLike for &TextureHandle {
 /// What a widget declares a length of its box to be. `leftover` is not one: a
 /// share of what is left over is only a length to the widget dividing one,
 /// so it passes up in the size instead.
-pub(crate) fn declared_len(widget: &dyn Widget, axis: Axis) -> Option<Len> {
-    widget.size_hint(axis).filter(|len| len.leftover == 0.0)
+pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<Len>; 2] {
+    let rules = widgets.size_rules(id);
+    let widget = widgets.get_dyn(id);
+    AXES.map(|axis| {
+        rules.axis(axis).declared().or_else(|| {
+            // A hint still narrows the box where no rule does, which is how a
+            // widget with a natural pixel size -- an image, a gap -- gets that
+            // size rather than the whole offer. That is the offer's business
+            // rather than a declaration's, and this falls away once a widget
+            // occupies its reported size inside the box it was offered.
+            widget
+                .and_then(|widget| widget.size_hint(axis))
+                .filter(|len| len.leftover == 0.0)
+        })
+    })
 }
 
 /// Takes a widget's declared lengths in the box `region` is given in, since a