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/render_state.rs

@@ -3,8 +3,8 @@ use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::ui::painter::{declared_box, declared_lens, placed_box};
 use crate::{
     ActiveData, Axis, DrawLayers, Holds, IdLike, Len, MaskIdx, MoveIdx, Moves, Painter,
-    PixelRegion, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, WidgetId,
-    Widgets,
+    PixelRegion, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiScalar, UiSpan, Weight,
+    WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
 
@@ -245,10 +245,11 @@ impl UiRenderState {
         let mut settled = answer;
         for axis in AXES {
             let reported = answer.0.axis(axis);
-            let placed_len = match reported.leftover != 0.0 || declared[axis as usize].is_some() {
-                true => UiScalar::FULL,
-                false => UiScalar::new(reported.rel, reported.px),
-            };
+            let placed_len =
+                match reported.leftover != Weight::ZERO || declared[axis as usize].is_some() {
+                    true => UiScalar::FULL,
+                    false => UiScalar::from_parts(reported.rel, reported.px),
+                };
             settled.1[axis as usize] =
                 settled.1[axis as usize].and(drawing_holds[axis as usize].through(placed_len));
         }