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/layout.rs

@@ -131,22 +131,22 @@ fn a_moved_subtree_takes_its_children_with_it() {
 fn a_fixed_length_child_keeps_it_when_the_box_around_it_grows() {
     let mut h = Harness::new((400, 200));
     let fixed = rect(Color::BLUE).width(50).add(&mut h.rsc);
-    let rest = rect(Color::GREEN).add(&mut h.rsc);
-    let panel = (fixed, rest).span(Dir::RIGHT).add(&mut h.rsc);
+    let leftover = rect(Color::GREEN).add(&mut h.rsc);
+    let panel = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
     // Changing the bar's width is the only thing that changes the box the
     // panel and everything under it was drawn for.
     let bar = rect(Color::RED).width(100).add(&mut h.rsc);
     h.set_root((bar, panel).span(Dir::RIGHT));
     assert_corners!(h, fixed, (100, 0), (150, 200));
-    assert_corners!(h, rest, (150, 0), (400, 200));
+    assert_corners!(h, leftover, (150, 0), (400, 200));
 
     h.rsc[bar].x = Some(Len::px(200));
     h.frame();
 
     // The panel's box is 100 shorter, so the fixed child is the same 50 wide
-    // against its new start and the one taking the rest absorbs the change.
+    // against its new start and the one taking what is left absorbs the change.
     assert_corners!(h, fixed, (200, 0), (250, 200));
-    assert_corners!(h, rest, (250, 0), (400, 200));
+    assert_corners!(h, leftover, (250, 0), (400, 200));
 }
 
 #[test]
@@ -187,7 +187,7 @@ fn only_a_container_that_places_its_children_lengthens_the_chain() {
     );
 }
 
-/// A span that sizes from its children passes their `rest` weight up rather
+/// A span that sizes from its children passes their `leftover` weight up
 /// than collapsing it to one share, so nesting divides the same space instead
 /// of re-dividing a share of it.
 #[test]
@@ -256,7 +256,7 @@ fn hairline(h: &mut Harness, marks: &mut Vec<WidgetId>) -> StrongWidget {
 fn share(h: &mut Harness, inner: StrongWidget, ratio: f32) -> StrongWidget {
     SetSize {
         inner,
-        x: Some(Len::rest(ratio)),
+        x: Some(Len::leftover(ratio)),
         y: None,
     }
     .add_strong(&mut h.rsc)