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/active.rs

@@ -1,5 +1,5 @@
 use crate::{
-    Holds, LayerId, Len, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
+    Holds, LayerId, LayoutLen, MaskIdx, MoveIdx, PrimitiveHandle, RegionAlign, Size, TextureHandle,
     UiRegion, WidgetId,
 };
 
@@ -40,7 +40,7 @@ pub struct ActiveData {
     /// The declared lengths whoever drew this widget resolved into its box.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
-    pub declared: [Option<Len>; 2],
+    pub declared: [Option<LayoutLen>; 2],
     /// The alignment its parent asked it with. A local redraw repeats that
     /// question, including an override chosen by a container.
     pub align: RegionAlign,