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>

tests/generated.rs

@@ -178,7 +178,7 @@ fn reshuffle(
 /// it has to be buildable from what the failure printed.
 fn describe(id: WidgetId, h: &Harness) -> String {
     let rules = h.rsc.widgets().size_rules(id);
-    let rule = |r: SizeRule| match r.known() {
+    let rule = |r: SizeRule| match r.exact() {
         Some(len) => format!("{len}"),
         None => "-".into(),
     };