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/cases/retained.rs

@@ -125,10 +125,7 @@ fn moving_an_ordinary_subtree_remaps_its_mask() {
     let active = &h.render.active[&masked.id()];
     assert_eq!(
         h.rsc.ui().masks[active.mask.idx()].region,
-        UiRegion::new(
-            UiSpan::new(UiScalar::px(150.0), UiScalar::rel_max()),
-            UiSpan::FULL,
-        )
+        UiRegion::new(UiSpan::new(Len::px(150.0), Len::rel_max()), UiSpan::FULL,)
     );
     assert_corners!(h, inner, (150, 0), (400, 200));
 }
@@ -312,7 +309,7 @@ fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
 
     assert_eq!(draws.get(), settled);
     assert_corners!(h, leaf, (0, 0), (400, 100));
-    assert_eq!(h.render.active[&span.id()].size.y, Len::rel(1.0));
+    assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
 }
 
 /// The output is the root of the box chain, so a resize is a box that changed
@@ -407,12 +404,12 @@ fn a_box_change_within_one_step_is_not_a_change() {
 
     let step = Px::STEP.to_f32();
     for part in [0.1, 0.2, 0.3] {
-        h.rsc[first].size.x = Len::px(100.0 + step * part);
+        h.rsc[first].size.x = LayoutLen::px(100.0 + step * part);
         h.frame();
         assert_eq!(draws.get(), settled);
     }
 
-    h.rsc[first].size.x = Len::px(100.0 + step);
+    h.rsc[first].size.x = LayoutLen::px(100.0 + step);
     h.frame();
     assert_eq!(draws.get(), settled + 1);
 }