Rename rest to leftover

The length kind that asks for a part of what is left once the fixed
lengths are taken is called leftover: Len::leftover(2), Len::LEFTOVER,
Size::LEFTOVER, Len::leftover the field, and apply_leftover. It says
what it is where "rest" reads as "the remainder of the list" as often as
"the remaining space", and every agent who has touched this has reached
for a third word for it.

Locals called rest that meant a region or a widget are renamed with it,
since the word now names something else.

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

tests/shrink.rs

@@ -327,7 +327,7 @@ fn sized(rng: &mut Rng, inner: Node) -> Node {
     }
     let len = |rng: &mut Rng| match rng.below(4) {
         0 => Some(Len::px(20.0 + rng.below(180) as f32)),
-        1 => Some(Len::REST),
+        1 => Some(Len::LEFTOVER),
         _ => None,
     };
     Node::Sized(len(rng), len(rng), Box::new(inner))
@@ -343,7 +343,7 @@ fn grow(rng: &mut Rng, depth: usize) -> Node {
     }
     let len = |rng: &mut Rng| match rng.below(4) {
         0 => Some(Len::px(20.0 + rng.below(180) as f32)),
-        1 => Some(Len::REST),
+        1 => Some(Len::LEFTOVER),
         2 => Some(Len::rel(0.25 + rng.below(3) as f32 * 0.25)),
         _ => None,
     };