Carry a span's rest weight up instead of collapsing it to one share

A span reporting `Len::default()` whenever a child had a share threw away
how many shares it was holding, so each level of nesting re-divided a
share rather than dividing the same space. One span of a rect beside a
span of three gave 1/2 and 1/6 each, where the same four rects directly
in one span get a quarter.

A span that sizes from its children does not resolve `rest`, it passes
the weight up; resolution belongs at the nearest ancestor with a length,
and since the output became a box there is always one. The placement loop
already divides by `len.rest / total.rest`, so it consumes carried
weights unchanged -- only what the span reported was wrong.

The uneven nesting is the case that fails without this; the even one
passes either way and is here as the statement of intent.

Decided by the owner, 2026-09-14.

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

tests/layout.rs

@@ -186,3 +186,46 @@ fn only_a_container_that_places_its_children_lengthens_the_chain() {
         "the span above the leaf, and the root the window is held in"
     );
 }
+
+/// A span that sizes from its children passes their `rest` weight up rather
+/// than collapsing it to one share, so nesting divides the same space instead
+/// of re-dividing a share of it.
+#[test]
+fn nested_spans_divide_the_space_once_however_deep_the_nesting_is() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let left = (a, b).span(Dir::RIGHT).add(&mut h.rsc);
+    let right = (c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((left, right).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}
+
+/// The same space, unevenly nested: weights carried up mean a share is a
+/// share of the whole, not of whatever branch a widget happens to sit in.
+#[test]
+fn an_uneven_nesting_still_gives_every_share_the_same_length() {
+    let mut h = Harness::new((400, 200));
+    let (a, b, c, d) = (
+        rect(Color::RED).add(&mut h.rsc),
+        rect(Color::BLUE).add(&mut h.rsc),
+        rect(Color::GREEN).add(&mut h.rsc),
+        rect(Color::WHITE).add(&mut h.rsc),
+    );
+    let one = (a,).span(Dir::RIGHT).add(&mut h.rsc);
+    let three = (b, c, d).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((one, three).span(Dir::RIGHT));
+
+    for (i, id) in [a, b, c, d].into_iter().enumerate() {
+        let x = i as f32 * 100.0;
+        assert_corners!(h, id, (x, 0), (x + 100.0, 200));
+    }
+}