Sum a span's cursor by name, and guard a scroll's re-clamp

`cursor` added `px` and `rel` by hand where the placing loop below now says
`fixed += len.without_leftover()` -- the same sum, one of them named. And
`let along = total` shadowed the closure that makes a span along the row,
two meanings for one word in one function; the local said nothing `total`
did not.

A scroll's draw writes `amt` and `snap_end`, so a second draw at another
viewport reads what the first wrote. Warm still matches cold because
re-clamping is idempotent, but nothing said so and nothing checked it: the
seed scans build scrolls and never scroll them. The test scrolls four
distances, one past the end, and widens.

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

src/widget/position/span.rs

@@ -37,8 +37,8 @@ impl Widget for Span {
                     painter.widget_at(child, room).len(axis)
                 }
             };
-            cursor.px += len.px + self.gap;
-            cursor.rel += len.rel;
+            cursor += len.without_leftover();
+            cursor.px += self.gap;
             lens.push(len);
         }
 
@@ -146,12 +146,11 @@ impl Widget for Span {
         // get a quarter each, which collapsing to `leftover(1)` per level does
         // not give. Resolution happens at the nearest ancestor with a length,
         // and the root always has one.
-        let along = total;
         let ortho = match shrinks {
             true => ortho,
             false => LayoutLen::rel(1.0),
         };
-        Size::from_axis(axis, along, ortho)
+        Size::from_axis(axis, total, ortho)
     }
 }