Do not multiply by a part of nothing

`lerp` is `a + (b - a) * f`, and `b - a` is nothing often enough to be worth
asking: a box with the same pixels at both ends of an axis, a span with no
fraction of one, a part of a subtree whose box did not move on that axis.
`Fixed::scaled` is `mul` that answers a zero receiver without widening to
`i64`, rounding and narrowing back, and `lerp` uses it -- so every lerp in
layout gets it rather than the two places that were about to grow their own
comparison.

`many` over 500 frames: 1,705,786,553 instructions to 1,657,571,216, and
638.9M cycles against 657.9M, averaged over four runs each.

Checked: fmt, clippy, 105 tests, all five shrinker cases at 300 seeds, and
`tabs`, `text`, `random`, `minimal` and `view` byte-identical at 1920x1200.

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

core/src/fixed.rs

@@ -138,6 +138,16 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         Self(narrow(shift_round(self.0 as i64 * by.0 as i64, BY)))
     }
 
+    /// A part of a span that is often nothing: no part of nothing is
+    /// nothing, for the cost of a comparison rather than a widening
+    /// multiply and a rounding.
+    pub const fn scaled<const BY: u32>(self, by: Fixed<BY>) -> Self {
+        match self.0 == 0 {
+            true => self,
+            false => self.mul(by),
+        }
+    }
+
     /// Repeated a whole number of times, which no grid rounds.
     pub const fn mul_int(self, by: i32) -> Self {
         Self(narrow(self.0 as i64 * by as i64))
@@ -179,7 +189,7 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// `from` and `to` a fraction of the way apart, the fraction being the
     /// receiver -- the argument order [`crate::util::LerpUtil`] already uses.
     pub const fn lerp<const OF: u32>(self, from: Fixed<OF>, to: Fixed<OF>) -> Fixed<OF> {
-        from.add(to.sub(from).mul(self))
+        from.add(to.sub(from).scaled(self))
     }
 
     pub const fn min(self, other: Self) -> Self {