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/render_state.rs

@@ -2,9 +2,9 @@
 use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::ui::painter::{declared_box, declared_lens, placed_box};
 use crate::{
-    ActiveData, Axis, DrawLayers, Holds, IdLike, Len, MaskIdx, MoveIdx, Moves, Painter,
-    PixelRegion, PxVec2, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan,
-    Weight, WidgetId, Widgets,
+    ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutLen, Len, MaskIdx, MoveIdx, Moves, Painter,
+    PixelRegion, PxVec2, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan, Weight,
+    WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -75,14 +75,8 @@ impl UiRenderState {
     /// downstream has to know the output's size to resolve a position.
     fn write_root(&mut self) {
         let region = UiRegion::new(
-            UiSpan::new(
-                UiScalar::ZERO,
-                UiScalar::from_parts(Rel::ZERO, self.output_size.x),
-            ),
-            UiSpan::new(
-                UiScalar::ZERO,
-                UiScalar::from_parts(Rel::ZERO, self.output_size.y),
-            ),
+            UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, self.output_size.x)),
+            UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, self.output_size.y)),
         );
         match self.root_move == MoveIdx::NONE {
             true => self.root_move = self.moves.push(MoveIdx::NONE, region),
@@ -255,8 +249,8 @@ impl UiRenderState {
             let reported = answer.0.axis(axis);
             let placed_len =
                 match reported.leftover != Weight::ZERO || declared[axis as usize].is_some() {
-                    true => UiScalar::FULL,
-                    false => UiScalar::from_parts(reported.rel, reported.px),
+                    true => Len::FULL,
+                    false => Len::from_parts(reported.rel, reported.px),
                 };
             settled.1[axis as usize] =
                 settled.1[axis as usize].and(drawing_holds[axis as usize].through(placed_len));
@@ -765,10 +759,10 @@ impl UiRenderState {
         let size = Size {
             x: widget
                 .and_then(|w| w.size_hint(Axis::X))
-                .unwrap_or(Len::ZERO),
+                .unwrap_or(LayoutLen::ZERO),
             y: widget
                 .and_then(|w| w.size_hint(Axis::Y))
-                .unwrap_or(Len::ZERO),
+                .unwrap_or(LayoutLen::ZERO),
         };
         self.active.insert(
             id,
@@ -1079,7 +1073,7 @@ impl RegionRemap {
         }
     }
 
-    fn apply_scalar(self, scalar: UiScalar, from: UiSpan, to: UiSpan) -> UiScalar {
+    fn apply_scalar(self, scalar: Len, from: UiSpan, to: UiSpan) -> Len {
         let extent = from.end.rel - from.start.rel;
         // A box that only moved, or that has no relative extent to divide,
         // carries its parts by moving them, which is exact. Dividing to find
@@ -1100,7 +1094,7 @@ impl RegionRemap {
         let from_px = fraction.lerp(from.start.px, from.end.px);
         let to_rel = fraction.lerp(to.start.rel, to.end.rel);
         let to_px = fraction.lerp(to.start.px, to.end.px);
-        UiScalar::from_parts(to_rel, scalar.px - from_px + to_px)
+        Len::from_parts(to_rel, scalar.px - from_px + to_px)
     }
 }