Put lengths, padding, gaps and alignment on the grid too

`Len` is `Px` beside `Rel` beside `Weight`, so the seam `4e28f10` left in
`Span` -- a float length added to a fixed-point cursor -- is gone, and the
sum a span compares against its box is exact.

`Weight` is its own scale, `Fixed<16>`, because a share of what is left over
is not a fraction of anything: a list divides its room by the total of them,
so the range has to hold a whole list's worth while the precision only has to
tell two weights apart. `Rel::ratio` turns two weights into a share on the
finer grid, which is what a span needs and what dividing them on their own
grid would round away.

`AxisAlign` holds a `Rel` rather than a float, which is what the layout was
reading out of it anyway. `Padding` and `Span::gap` hold `Px`, converted
where they are built instead of on every frame. `RegionAlign::rel` is gone;
its one caller wanted a position, and now builds one.

`Fixed` gains `from_num` for a number as it is written in source, `mul_int`
for a length repeated a whole number of times, and `ratio`.

Checked: fmt, clippy, 101 tests, 100 generated seeds in 86 s, all five
shrinker cases at 300 seeds, and all five examples byte-identical at
1920x1200 against `4e28f10`.

With the fuzzer comparing for equality rather than within 0.05 px, four of
the five cases now pass 100 seeds -- `resize-repaint` joins the other three.
`reorder` still fails one seed by one step, so the last of it is in what a
box is measured *in*: `px_len` and the window are still floats.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/orientation/align.rs

@@ -1,4 +1,4 @@
-use crate::{Px, Rel, vec2};
+use crate::{Px, Rel};
 
 use super::*;
 
@@ -35,7 +35,7 @@ impl Align {
 /// is the near one depends on the writing system and on which way a container
 /// runs, and the middle is the same either way.
 #[derive(Debug, Clone, Copy, PartialEq)]
-pub struct AxisAlign(f32);
+pub struct AxisAlign(Rel);
 
 impl AxisAlign {
     pub const NEG: Self = Self::new(0.0);
@@ -43,10 +43,12 @@ impl AxisAlign {
     pub const POS: Self = Self::new(1.0);
 
     pub const fn new(rel: f32) -> Self {
-        Self(rel)
+        Self(Rel::from_f32(rel))
     }
 
-    pub const fn rel(&self) -> f32 {
+    /// A fraction of the room left over, which is what the layout reads: the
+    /// three constants are the familiar places along it, not the only ones.
+    pub const fn rel(&self) -> Rel {
         self.0
     }
 }
@@ -118,9 +120,6 @@ impl RegionAlign {
     pub const fn new(x: AxisAlign, y: AxisAlign) -> Self {
         Self { x, y }
     }
-    pub const fn rel(&self) -> Vec2 {
-        vec2(self.x.rel(), self.y.rel())
-    }
 }
 
 impl UiVec2 {
@@ -175,7 +174,7 @@ impl Vec2 {
 
 impl UiScalar {
     pub const fn align(&self, align: AxisAlign) -> UiSpan {
-        let rel = Rel::from_f32(align.rel());
+        let rel = align.rel();
         let rest = Rel::ONE.sub(rel);
         let at = UiScalar::from_parts(rel, Px::ZERO);
         UiSpan {
@@ -221,7 +220,10 @@ impl From<CardinalAlign> for Align {
 
 const impl From<RegionAlign> for UiVec2 {
     fn from(align: RegionAlign) -> Self {
-        Self::rel(align.rel())
+        Self::new(
+            UiScalar::from_parts(align.x.rel(), Px::ZERO),
+            UiScalar::from_parts(align.y.rel(), Px::ZERO),
+        )
     }
 }