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>

tests/shrink.rs

@@ -119,7 +119,7 @@ impl Node {
                 let handle = Span {
                     children,
                     dir: dir(*down),
-                    gap: *gap,
+                    gap: Px::from_f32(*gap),
                     ortho: match down {
                         true => OrthoSize::Children,
                         false => OrthoSize::Full,
@@ -140,12 +140,7 @@ impl Node {
             Node::Pad(p, kid) => {
                 let inner = kid.build(h, out, spans, sized);
                 Pad {
-                    padding: Padding {
-                        left: *p,
-                        right: *p,
-                        top: *p,
-                        bottom: *p,
-                    },
+                    padding: Padding::uniform(*p),
                     inner,
                 }
                 .add_strong(&mut h.rsc)
@@ -407,9 +402,10 @@ enum Case {
 /// A different declared length, kept the same kind so the change is to the
 /// value alone.
 fn resized_len(len: Option<Len>) -> Option<Len> {
+    let half = Rel::from_f32(0.5);
     len.map(|len| Len {
-        px: len.px * 0.5 + 13.0,
-        rel: len.rel * 0.5,
+        px: len.px.mul(half) + Px::from_int(13),
+        rel: len.rel.mul(half),
         leftover: len.leftover,
     })
 }