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/ui/holds.rs

@@ -1,4 +1,4 @@
-use crate::{Px, REL_SHIFT, UiScalar, fixed::div_toward, fixed::narrow};
+use crate::{Len, Px, REL_SHIFT, fixed::div_toward, fixed::narrow};
 use std::ops::RangeInclusive;
 
 /// The lengths of a box, in pixels, that one drawing of a widget holds for:
@@ -50,7 +50,7 @@ impl Holds {
     /// the length alone instead gives a point that need not even contain the
     /// box the part was drawn in, which is a range excluding the drawing it
     /// was made for.
-    pub const fn through(self, len: UiScalar) -> Self {
+    pub const fn through(self, len: Len) -> Self {
         let rel = len.rel.raw() as i64;
         if rel == 0 {
             return Self::ANY;
@@ -98,7 +98,7 @@ mod tests {
     #[test]
     fn through_reverses_a_range_for_a_negative_fraction() {
         // `10 - box / 2` is between 20 and 40 for boxes from -60 to -20.
-        let part = UiScalar::from_parts(Rel::from_f32(-0.5), Px::from_int(10));
+        let part = Len::from_parts(Rel::from_f32(-0.5), Px::from_int(10));
         let holds = Holds::from(Px::from_int(20)..=Px::from_int(40)).through(part);
         assert!(holds.contains(Px::from_int(-60)) && holds.contains(Px::from_int(-20)));
         assert!(!holds.contains(Px::from_int(-61)) && !holds.contains(Px::from_int(-19)));
@@ -109,7 +109,7 @@ mod tests {
     /// box is not a whole number of steps.
     #[test]
     fn a_part_maps_back_onto_the_box_it_was_measured_in() {
-        let part = UiScalar::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
+        let part = Len::from_parts(Rel::from_f32(1.0 / 3.0), Px::from_int(-146));
         for box_len in (440..460).map(Px::from_int) {
             let holds = Holds::at(part.to_px(box_len)).through(part);
             assert!(holds.contains(box_len), "{box_len:?} left out by {holds:?}");