Measure a child on the layer it draws on, not twice on two

Fixed point cost 3x in layout: `many` went from 0.179 ms a frame to 0.544,
and `scroll` from 0.011 to 0.030. The counters said why -- eight more "placed
by redrawing" a frame -- and the reason was mine rather than the grid's. A
retained drawing belongs to the layer it was made on, which `4e28f10` started
enforcing, and `Stack` measures the child that sizes it by drawing it on its
own layer and then draws it again on the child layer. So every stacked child
redrew twice a frame, forever.

`Painter::child_layer_at` addresses a child's layer rather than walking to
it, and `Stack` measures on the layer that child ends up on. The second ask
is then a reuse. Its glyphs are written once rather than once under the
background and once over it.

Measured on the same fixture: `scroll` 0.031 ms to 0.020, `many` 0.570 to
0.283, and the scroll phase's counters are back to what they were before
fixed point -- 4 widget draws and 12 draw requests a frame, exactly. What is
left above that baseline is not this.

`ReuseOutcome` could not say "another layer" or "the region-node choice
changed"; both returned without a counter, which is why the first look at
this said nothing. They have counters now.

Checked: fmt, clippy, 105 tests, five shrinker cases at 300 seeds, 100
generated seeds, and the examples byte-identical but for 36 pixels of
`random` at one level -- edges that were being drawn twice.

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

core/src/layout_diagnostics.rs

@@ -40,6 +40,8 @@ pub(crate) enum Counter {
     ReuseWrongParent,
     ReuseRemapped,
     ReuseOutside,
+    ReuseWrongLayer,
+    ReuseWrongNode,
     PlaceRedraws,
     QueuePops,
     DepthReads,
@@ -73,6 +75,8 @@ impl Counter {
         "reuse: wrong parent",
         "reuse remapped",
         "reuse: outside what it holds for",
+        "reuse: another layer",
+        "reuse: region-node choice changed",
         "placed by redrawing",
         "redraw queue pops",
         "depth reads",