Answer "have I asked this child?" in one read

A container's draw asked it once per child by searching the list of children
it had added so far, and four other per-child steps searched a list too, so
one draw cost the square of its children: 70% of a 1,600-child redraw was
those searches. A draw takes a DrawId and leaves it on every widget it asks
about; one note per widget is enough because the handle a container holds a
child by cannot be cloned. tests/children_cost.rs is the rig that shows it,
and it is the only one here that varies width: 3.680 ms to 0.811 ms at 1,600
children, and flat per child.

Beside it, the rest of the fourteenth sweep of #19: a mask's rectangle
resolved once per fragment instead of once per instance, which takes the
storage buffers out of the fragment stage and is 8.8x on a screenful of
deeply nested clips; TextBuffer::shape copying its attrs before the check
that would not need them, which allocated once per named-family text per
frame; a should_panic test on a debug assertion that made cargo test
--release fail; Fixed::div, reached only by its own test; Moves::remove
re-uploading an array it cannot have changed; and two comments the branch
itself falsified.

docs/LAYOUT_LOG.md has all eight with their measurements, the five things
looked at and left, and what was verified.

tests/cases/scroll.rs

@@ -127,6 +127,11 @@ fn wrapping_content_beside_a_fixed_length_is_stable_warm_and_cold() {
 /// parent would place the part it cut off, and the framework would put a
 /// drawing longer than its box somewhere. `Masked` is the second of these
 /// after `Scroll`, and the assertion in `draw_at` is what says so.
+// What it checks is a debug assertion, which a release build does not compile
+// -- and a `should_panic` test of one fails there rather than passing
+// vacuously, so it is not built either. Every measurement rig here is run in
+// release, so `cargo test --release` has to pass.
+#[cfg(debug_assertions)]
 #[test]
 #[should_panic = "clips to"]
 fn a_clipping_widget_reporting_more_than_its_box_is_caught() {