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>

core/src/orientation/align.rs

@@ -172,14 +172,14 @@ impl Vec2 {
     }
 }
 
-impl UiScalar {
+impl Len {
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
         let rel = align.rel();
         let rest = Rel::ONE.sub(rel);
-        let at = UiScalar::from_parts(rel, Px::ZERO);
+        let at = Len::from_parts(rel, Px::ZERO);
         UiSpan {
-            start: UiScalar::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
-            end: UiScalar::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
+            start: Len::from_parts(at.rel.sub(self.rel.mul(rel)), at.px.sub(self.px.mul(rel))),
+            end: Len::from_parts(at.rel.add(self.rel.mul(rest)), at.px.add(self.px.mul(rest))),
         }
     }
 }
@@ -221,8 +221,8 @@ impl From<CardinalAlign> for Align {
 const impl From<RegionAlign> for UiVec2 {
     fn from(align: RegionAlign) -> Self {
         Self::new(
-            UiScalar::from_parts(align.x.rel(), Px::ZERO),
-            UiScalar::from_parts(align.y.rel(), Px::ZERO),
+            Len::from_parts(align.x.rel(), Px::ZERO),
+            Len::from_parts(align.y.rel(), Px::ZERO),
         )
     }
 }