Read a slot's ends where they are used, not carry one between children

`start` looked like a third accumulator beside `fixed` and `taken`, carried
across iterations and assigned at three points. It was never independent:
every assignment was `shared(fixed, taken)`, so it was those two read
together. Reading it at each end of a slot instead drops the variable, drops
two of the three calls per child, and leaves the gap added after the last
child deriving nothing -- which was the thing that read as a bug, and is not
one because no end is taken from it.

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

src/widget/position/span.rs

@@ -83,15 +83,14 @@ impl Widget for Span {
         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
-        // from the last child: the share of the room is rounded, and taking
-        // each from the one before it would carry every rounding along the
-        // row.
+        // spoken for. Both ends of a slot are read from those two rather
+        // than stepped from the last child: the share of the room is
+        // rounded, and taking each end from the one before it would carry
+        // every rounding along the row.
         let mut fixed = Len::rel_min();
         let mut taken = Weight::ZERO;
-        let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        let shared = |fixed: Len, taken: Weight| match taken == Weight::ZERO {
+        let reached = |fixed: Len, taken: Weight| match taken == Weight::ZERO {
             true => fixed,
             false => fixed + room.scale(Rel::ratio(taken, total.leftover)),
         };
@@ -102,15 +101,14 @@ impl Widget for Span {
             if len.is_only_leftover() && !shares {
                 painter.undraw(child);
                 fixed.px += self.gap;
-                start = shared(fixed, taken);
                 continue;
             }
-            let from = start;
+            let from = reached(fixed, taken);
             if len.leftover > Weight::ZERO && shares {
                 taken += len.leftover;
             }
             fixed += len.without_leftover();
-            start = shared(fixed, taken);
+            let to = reached(fixed, taken);
             // Along the row the span says where the child goes, and that slot
             // is the child's box outright rather than something to place an
             // answer inside again. A share is decided here and nowhere
@@ -118,7 +116,7 @@ impl Widget for Span {
             // it, since a text wraps at the width it is actually given. A
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
-            let slot = along(from, start);
+            let slot = along(from, to);
             let mut place = slot.shifted_desc().fills().on_axis(axis);
             if len.leftover > Weight::ZERO && shares {
                 place = place.rel_base(axis, slot.len());
@@ -136,7 +134,6 @@ impl Widget for Span {
                 }
             }
             fixed.px += self.gap;
-            start = shared(fixed, taken);
         }
 
         // Carried whole rather than collapsed to one share: a span that sizes