Say on_axis in the docs, and point the handoff at the tip

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

docs/LAYOUT.md

@@ -371,7 +371,7 @@ wrong with nothing failing.
 `PlaceDesc` is the pair, with `x` and `y` fields and the `Index<Axis>` and
 `from_axis` of every other pair here, so the joint work — resolving a
 region, reading the fill flags — is written once rather than per axis.
-`PlaceDescAxis::axis(axis)` goes the other way, lifting one axis into a pair
+`PlaceDescAxis::on_axis(axis)` goes the other way, lifting one axis into a pair
 with the whole box across it.
 
 **Every pair here is a struct of two per-axis values, read with `[axis]`.**