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/orientation/pos.rs

@@ -6,41 +6,41 @@ use crate::{Px, PxVec2, Rel, UiNum, util::impl_op};
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, bytemuck::Zeroable, Default)]
 pub struct UiVec2 {
-    pub x: UiScalar,
-    pub y: UiScalar,
+    pub x: Len,
+    pub y: Len,
 }
 
 impl UiVec2 {
     pub const ZERO: Self = Self {
-        x: UiScalar::ZERO,
-        y: UiScalar::ZERO,
+        x: Len::ZERO,
+        y: Len::ZERO,
     };
 
-    pub const fn new(x: UiScalar, y: UiScalar) -> Self {
+    pub const fn new(x: Len, y: Len) -> Self {
         Self { x, y }
     }
 
     pub const fn px(px: impl const Into<Vec2>) -> Self {
         let px = px.into();
         Self {
-            x: UiScalar::px(px.x),
-            y: UiScalar::px(px.y),
+            x: Len::px(px.x),
+            y: Len::px(px.y),
         }
     }
 
     /// From lengths already on the grid, with no fraction of a box.
     pub const fn from_px(px: PxVec2) -> Self {
         Self {
-            x: UiScalar::from_parts(Rel::ZERO, px.x),
-            y: UiScalar::from_parts(Rel::ZERO, px.y),
+            x: Len::from_parts(Rel::ZERO, px.x),
+            y: Len::from_parts(Rel::ZERO, px.y),
         }
     }
 
     pub const fn rel(rel: impl const Into<Vec2>) -> Self {
         let rel = rel.into();
         Self {
-            x: UiScalar::rel(rel.x),
-            y: UiScalar::rel(rel.y),
+            x: Len::rel(rel.x),
+            y: Len::rel(rel.y),
         }
     }
 
@@ -61,14 +61,14 @@ impl UiVec2 {
         }
     }
 
-    pub fn axis_mut(&mut self, axis: Axis) -> &mut UiScalar {
+    pub fn axis_mut(&mut self, axis: Axis) -> &mut Len {
         match axis {
             Axis::X => &mut self.x,
             Axis::Y => &mut self.y,
         }
     }
 
-    pub fn axis(&self, axis: Axis) -> UiScalar {
+    pub fn axis(&self, axis: Axis) -> Len {
         match axis {
             Axis::X => self.x,
             Axis::Y => self.y,
@@ -83,7 +83,7 @@ impl UiVec2 {
 
     pub const FULL_SIZE: Self = Self::rel(Vec2::ONE);
 
-    pub const fn from_axis(axis: Axis, aligned: UiScalar, ortho: UiScalar) -> Self {
+    pub const fn from_axis(axis: Axis, aligned: Len, ortho: Len) -> Self {
         match axis {
             Axis::X => Self {
                 x: aligned,
@@ -129,21 +129,27 @@ where
     }
 }
 
-/// A position along one axis, as a fraction of the box it sits in plus an
-/// offset: `rel * len + px`. Both parts are fixed point, so composing one
-/// through a chain of boxes rounds only where it multiplies and lands on the
-/// same number as any other route to the same place.
+/// A length along one axis: a fraction of the box it is measured in plus an
+/// offset, `rel * box + px`. A position is the same number -- the length from
+/// the start of the box to the point -- which is why a [`UiSpan`] is two of
+/// these. Both parts are fixed point, so composing one through a chain of
+/// boxes rounds only where it multiplies, and lands on the same number as any
+/// other route to the same place.
+///
+/// It carries no claim on what a container has left over. That is
+/// [`crate::LayoutLen`], which is this plus a weight, and which means nothing
+/// to anyone but whoever divides the room.
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, bytemuck::Pod, Default, bytemuck::Zeroable)]
-pub struct UiScalar {
+pub struct Len {
     pub rel: Rel,
     pub px: Px,
 }
 
-impl_op!(same UiScalar Add add; rel px);
-impl_op!(same UiScalar Sub sub; rel px);
+impl_op!(same Len Add add; rel px);
+impl_op!(same Len Sub sub; rel px);
 
-impl UiScalar {
+impl Len {
     pub const ZERO: Self = Self {
         rel: Rel::ZERO,
         px: Px::ZERO,
@@ -192,10 +198,8 @@ impl UiScalar {
         }
     }
 
-    /// Both channels by the same factor, which is what a fraction of a
-    /// length means when the length is part pixels and part a share.
-    /// Both channels by the same fraction, which is what a part of a length
-    /// means when the length is part pixels and part a share.
+    /// Both parts by the same fraction, which is what a part of a length
+    /// means when the length is part pixels and part a fraction of a box.
     pub const fn scale(&self, by: Rel) -> Self {
         Self {
             rel: self.rel.mul(by),
@@ -215,14 +219,14 @@ impl UiScalar {
         }
     }
 
-    pub fn within_len(&self, len: UiScalar) -> Self {
+    pub fn within_len(&self, len: Len) -> Self {
         self.within(&UiSpan {
-            start: UiScalar::ZERO,
+            start: Len::ZERO,
             end: len,
         })
     }
 
-    pub fn select_len(&self, len: UiScalar) -> Self {
+    pub fn select_len(&self, len: Len) -> Self {
         len.within_len(*self)
     }
 
@@ -245,24 +249,24 @@ impl UiScalar {
 #[repr(C)]
 #[derive(Debug, Copy, Clone, PartialEq, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct UiSpan {
-    pub start: UiScalar,
-    pub end: UiScalar,
+    pub start: Len,
+    pub end: Len,
 }
 
 impl UiSpan {
     pub const FULL: Self = Self {
-        start: UiScalar::ZERO,
-        end: UiScalar::FULL,
+        start: Len::ZERO,
+        end: Len::FULL,
     };
 
     pub const fn rel(rel: f32) -> Self {
         Self {
-            start: UiScalar::rel(rel),
-            end: UiScalar::rel(rel),
+            start: Len::rel(rel),
+            end: Len::rel(rel),
         }
     }
 
-    pub const fn new(start: UiScalar, end: UiScalar) -> Self {
+    pub const fn new(start: Len, end: Len) -> Self {
         Self { start, end }
     }
 
@@ -273,7 +277,7 @@ impl UiSpan {
         std::mem::swap(&mut self.start.px, &mut self.end.px);
     }
 
-    pub const fn shift(&mut self, offset: UiScalar) {
+    pub const fn shift(&mut self, offset: Len) {
         self.start += offset;
         self.end += offset;
     }
@@ -285,7 +289,7 @@ impl UiSpan {
         }
     }
 
-    pub const fn len(&self) -> UiScalar {
+    pub const fn len(&self) -> Len {
         self.end - self.start
     }
 }