Renumber chapters 15 to 21 up by one, to open a slot for collision

A pure rename plus link rewrite and nothing else. Collision is a pluggable
subsystem with two implementations, shapes, a response hook and a tile binding;
that is a chapter, and burying it inside the physics chapter is the shape this
manual otherwise avoids -- rendering and drawing are 8 and 9, spritesheets and
characters and actors are 10, 11 and 12.

Kept separate from writing the chapter because **nothing validates a
cross-reference target.** The example harness reads fenced blocks; it does not
follow links. A rename mixed into five hundred lines of new prose is not
reviewable, and a broken link would land silently. As its own commit it is
reviewable as a rename, and a grep for dangling `](NN-` targets is clean.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/06-the-registry.md

@@ -224,5 +224,5 @@ registry's.
 - [Chapter 7](07-the-game-and-the-frame.md) — where in startup the registries are created,
   and where configuration has to be in place by.
 - [Chapter 14](14-physics.md) — choosing a backend, and what the `physics.*` properties do.
-- [Chapter 21](21-appendix-limits.md) — the configuration table again, alongside every
+- [Chapter 22](22-appendix-limits.md) — the configuration table again, alongside every
   `AKGL_MAX_*`.