Compose a position in the shader the way the CPU composes it

The shader used mix() where UiScalar::within writes from + (to - from) *
t, so the two associate the arithmetic differently and can put an edge on
either side of an integer. Writing it out matches them, and is a
multiplication cheaper. The five reference renders and the resize render
are byte-identical either way.

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

core/src/render/shader/prelude.wgsl

@@ -37,10 +37,13 @@ const MOVE_NONE: u32 = 4294967295u;
 // resolve a deep one the same way.
 const CHAIN_LIMIT: u32 = 64u;
 
+// Written the way `UiScalar::within` writes it rather than as `mix`, so the
+// CPU and the shader compose a position with the same arithmetic and answer
+// the same thing about where a widget is.
 fn scalar_within(s: UiScalar, p: UiSpan) -> UiScalar {
     return UiScalar(
-        mix(p.start.rel, p.end.rel, s.rel),
+        p.start.rel + (p.end.rel - p.start.rel) * s.rel,
-        s.px + mix(p.start.px, p.end.px, s.rel),
+        s.px + (p.start.px + (p.end.px - p.start.px) * s.rel),
     );
 }