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/random.rs

@@ -9,7 +9,7 @@ use crate::prelude::*;
 use std::collections::HashMap;
 
 /// The declared lengths of one widget carrying a size rule, by axis.
-pub type Lens = [Option<Len>; 2];
+pub type Lens = [Option<LayoutLen>; 2];
 
 /// Where one widget carrying an alignment sits, by axis. `None` uses the
 /// centered default.
@@ -181,10 +181,10 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         id
     }
 
-    fn len(&mut self) -> Option<Len> {
+    fn len(&mut self) -> Option<LayoutLen> {
         match self.rng.below(4) {
-            0 => Some(Len::px(20.0 + self.rng.below(180) as f32)),
-            1 => Some(Len::LEFTOVER),
+            0 => Some(LayoutLen::px(20.0 + self.rng.below(180) as f32)),
+            1 => Some(LayoutLen::LEFTOVER),
             _ => None,
         }
     }
@@ -367,7 +367,7 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
         if dir.axis == Axis::X {
             self.rsc
                 .widgets_mut()
-                .set_size_rules(id, None, Some(Len::rel(1.0)));
+                .set_size_rules(id, None, Some(LayoutLen::rel(1.0)));
         }
         self.tree.ids.push(id.id());
         self.tree.spans.push(Spanned { id, spares, grown });