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/ui/painter.rs

@@ -1,8 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, Holds, Len, Px, RegionAlign, RenderedText, Size, StrongWidget, TextAttrs, TextBuffer,
-    TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, WidgetId, Widgets,
+    Axis, Holds, Len, Px, RegionAlign, Rel, RenderedText, Size, StrongWidget, TextAttrs,
+    TextBuffer, TextData, TextureHandle, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2, Weight,
+    WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveHandle, PrimitiveInst,
         PrimitiveKind, TexturePrimitive,
@@ -484,7 +485,7 @@ pub(crate) fn declared_lens(widgets: &Widgets, id: WidgetId) -> [Option<Len>; 2]
             // occupies its reported size inside the box it was offered.
             widget
                 .and_then(|widget| widget.size_hint(axis))
-                .filter(|len| len.leftover == 0.0)
+                .filter(|len| len.leftover == Weight::ZERO)
         })
     })
 }
@@ -509,11 +510,11 @@ pub(crate) fn placed_box(
     let mut placed = region;
     for (axis, declared) in AXES.into_iter().zip(declared) {
         let reported = size.axis(axis);
-        if reported.leftover != 0.0 || declared.is_some() {
+        if reported.leftover != Weight::ZERO || declared.is_some() {
             continue;
         }
         let span = placed.axis_mut(axis);
-        let len = span.len().scale(reported.rel) + UiScalar::px(reported.px);
+        let len = span.len().scale(reported.rel) + UiScalar::from_parts(Rel::ZERO, reported.px);
         span.start += (span.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
@@ -532,7 +533,7 @@ pub(crate) fn declared_box(
     for (axis, len) in AXES.into_iter().zip(declared) {
         let Some(len) = len else { continue };
         let span = region.axis_mut(axis);
-        let len = UiScalar::new(len.rel, len.px);
+        let len = UiScalar::from_parts(len.rel, len.px);
         span.start += (span.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }