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/widget/size_rule.rs

@@ -29,11 +29,12 @@ impl SizeRule {
         }
     }
 
-    /// The length this rule fixes, whether or not it can narrow a box. A
-    /// share is a length the widget's parent still has to divide, so it is
-    /// known here and resolved there -- unlike `declared`, which is only the
-    /// ones that give a box directly.
-    pub fn known(&self) -> Option<LayoutLen> {
+    /// The length this rule gives outright, whatever the widget reports --
+    /// which makes the widget's answer on that axis moot. A share counts: it
+    /// is a length the widget's parent still has to divide, so it is exact
+    /// here and resolved there, unlike `declared`, which is only the ones
+    /// that give a box directly.
+    pub fn exact(&self) -> Option<LayoutLen> {
         match self {
             Self::Free => None,
             Self::Exact(len) => Some(*len),