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/09-drawing.md

@@ -326,7 +326,7 @@ colour it found, which is also what makes it testable without a world.
 
 - [Chapter 08](08-rendering.md) — the backend these all take, and the frame the
   calls sit inside.
-- [Chapter 16](16-text-and-fonts.md) — text, which is textures rather than
+- [Chapter 17](17-text-and-fonts.md) — text, which is textures rather than
   primitives.
 - [Chapter 05](05-the-heap.md) — the pools, and why `copy_region` returning owned
   memory is the exception rather than the rule.