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/revision_cost.rs

@@ -94,7 +94,11 @@ fn build(h: &mut Harness, rows: usize) -> Vec<WidgetId> {
     let mut col = Span::empty(Dir::DOWN);
     for _ in 0..rows {
         let mut row = Span::empty(Dir::RIGHT);
-        row.push(rect(Color::RED).width(Len::px(40.0)).add_strong(&mut h.rsc));
+        row.push(
+            rect(Color::RED)
+                .width(LayoutLen::px(40.0))
+                .add_strong(&mut h.rsc),
+        );
         let mut body = Span::empty(Dir::DOWN);
         let para = wtext(words(&mut rng, 12, 52))
             .size(16)