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/layout_diagnostics.rs

@@ -15,7 +15,7 @@
 //! reuse, size, placement, and text events for one suspicious widget. The
 //! selection is a set and survives [`take`] until cleared.
 
-use crate::{Axis, Len, PxVec2, Size, UiRegion, WidgetId};
+use crate::{Axis, LayoutLen, PxVec2, Size, UiRegion, WidgetId};
 use std::{
     cell::RefCell,
     collections::{HashMap, HashSet},
@@ -287,7 +287,7 @@ pub enum TraceEvent {
         id: WidgetId,
         reader: WidgetId,
         axis: Axis,
-        hint: Option<Len>,
+        hint: Option<LayoutLen>,
     },
     TextRendered {
         id: WidgetId,
@@ -389,7 +389,7 @@ pub(crate) fn size_read(id: WidgetId, reader: WidgetId, size: Size) {
     trace(id, TraceEvent::SizeRead { id, reader, size });
 }
 
-pub(crate) fn hint_read(id: WidgetId, reader: WidgetId, axis: Axis, hint: Option<Len>) {
+pub(crate) fn hint_read(id: WidgetId, reader: WidgetId, axis: Axis, hint: Option<LayoutLen>) {
     trace(
         id,
         TraceEvent::HintRead {