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>

src/widget/position/scroll.rs

@@ -57,8 +57,8 @@ impl Widget for Scroll {
         if moved || self.content_len != self.container_len {
             let offset = UiVec2::from_axis(
                 self.axis,
-                UiScalar::from_parts(Rel::ZERO, anchor - self.amt),
-                UiScalar::ZERO,
+                Len::from_parts(Rel::ZERO, anchor - self.amt),
+                Len::ZERO,
             );
             region = region.offset(offset);
             region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);