Ask whether a rule gives the length, not whether there is one

`Painter::ruled` answered "is there a rule beside me on this axis", which is
the same question as "is my report moot" only while `Exact` is the only rule
there is. `Min`, `Max` and `Clamp` are queued, and under one of those the
answer is still the widget's to give and a span across itself still has to
read its children -- so the name would have been true and the meaning wrong,
which is the worst way for a predicate to age.

It is `has_exact_size` now, over `SizeRule::exact` rather than `known`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/ui/painter.rs

@@ -222,7 +222,7 @@ impl<'a> Painter<'a> {
         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(|| {
+        let hint = widgets.size_rules(id.id()).axis(axis).exact().or_else(|| {
             widgets
                 .get_dyn(id.id())
                 .and_then(|widget| widget.size_hint(axis))
@@ -360,17 +360,21 @@ impl<'a> Painter<'a> {
         self.rsc.widgets().alignment(self.id)
     }
 
-    /// Whether a rule beside this widget settles its length on `axis`, which
-    /// makes whatever it reports for that axis moot. The widget under a rule
-    /// does not otherwise learn of it -- this is for a container deciding
-    /// whether reading its children across an axis is worth anything, since
-    /// reading one is also what makes its own size depend on it.
-    pub fn ruled(&self, axis: Axis) -> bool {
+    /// Whether a rule beside this widget gives its length on `axis` outright,
+    /// which makes whatever it reports for that axis moot. A rule that only
+    /// bounds the length is not one of these: the answer is still the
+    /// widget's to give, and something still has to work it out.
+    ///
+    /// The widget under a rule does not otherwise learn of it -- this is for
+    /// a container deciding whether reading its children across an axis is
+    /// worth anything, since reading one is also what makes its own size
+    /// depend on it.
+    pub fn has_exact_size(&self, axis: Axis) -> bool {
         self.rsc
             .widgets()
             .size_rules(self.id)
             .axis(axis)
-            .known()
+            .exact()
             .is_some()
     }