Give a length with no share in it its own type again

`UiScalar` was `Len` without the `leftover` weight, which is the separation
canonical `main` already had as `Len` beside `LayoutLen` and this branch
collapsed. It is needed back for the queued clamp: a cap may not contain a
share, because a cap has to read the report a rule otherwise makes moot, and
a share puts the container's division into the same equation -- two
self-consistent assignments, which is the multiple-fixed-point failure
generated seed 13 punished for orthogonal sizing. `min(report, cap)` is not
a `LayoutLen` either: it is a sum of parts, and the smaller of two of them
is not one.

So `UiScalar` is `Len`, what was `Len` is `LayoutLen`, and the two say in
their docs which is which: a `Len` is pixels plus a fraction of a box -- a
position being the length from the box's start, which is why a span is two
of them -- and a `LayoutLen` is a `Len` plus a claim only a container
dividing its room can answer. `From<Len> for LayoutLen` is the one-way step
between them.

Names only; the shader's `UiScalar` is renamed with them. Checked: fmt,
clippy, 105 tests, and `tabs`, `minimal`, `view`, `text` and `random`
byte-identical at 1920x1200.

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

tests/cases/unsettled.rs

@@ -281,11 +281,11 @@ struct Wider {
 impl Widget for Wider {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         Size {
-            x: Len {
+            x: LayoutLen {
                 px: painter.px_len(Axis::X) + Px::from_f32(self.extra),
-                ..Len::ZERO
+                ..LayoutLen::ZERO
             },
-            y: Len::LEFTOVER,
+            y: LayoutLen::LEFTOVER,
         }
     }
 }
@@ -341,7 +341,7 @@ fn plant_boundary(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
     let whole = rect(Color::RED).add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_size_rules(whole, None, Some(Len::rel(1.0)));
+        .set_size_rules(whole, None, Some(LayoutLen::rel(1.0)));
     let mut inner_children: Vec<StrongWidget> = vec![
         measured.add_strong(&mut h.rsc),
         whole.add_strong(&mut h.rsc),
@@ -357,7 +357,7 @@ fn plant_boundary(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
     .add(&mut h.rsc);
     h.rsc
         .widgets_mut()
-        .set_size_rules(inner, None, Some(Len::px(198.0)));
+        .set_size_rules(inner, None, Some(LayoutLen::px(198.0)));
     // One more span above it: without a box composed through it, both trees
     // round the same way and the boundary is never crossed.
     let outer = (inner,).span(Dir::DOWN).add(&mut h.rsc);