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>

src/widget/position/span.rs

@@ -80,9 +80,11 @@ impl Widget for Span {
         }
 
         // Across itself a span is as long as its longest child -- unless a
-        // rule beside it already says how long it is, and then reading them
-        // answers nothing and makes its size depend on theirs for it.
-        let shrinks = !painter.ruled(!axis);
+        // rule beside it gives that length outright, and then reading them
+        // answers nothing and makes its size depend on theirs for it. A rule
+        // that only bounds the length does not count: the answer is still
+        // this span's to give.
+        let shrinks = !painter.has_exact_size(!axis);
         // What the fixed parts and the gaps before here take, which is a sum
         // of lengths and exact, and how much of the leftover weight is
         // spoken for. A position is one from the other rather than a step