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>

src/widget/position/scroll.rs

@@ -45,9 +45,12 @@ impl Widget for Scroll {
         // sits is this widget's own alignment -- the same property that would
         // have placed the whole scroll in a box longer than it.
         let slack = (self.container_len - self.content_len).max(0.0);
-        let anchor = slack * align.rel();
+        let anchor = slack * align.rel().to_f32();
         let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, anchor - self.amt, 0.0));
-        region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
+        region.axis_mut(self.axis).end = region
+            .axis(self.axis)
+            .start
+            .offset(Px::from_f32(self.content_len));
         painter.widget_aligned(&self.inner, region, RegionAlign::NEAR);
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for