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/03-getting-started.md

@@ -243,7 +243,7 @@ starts at 256, so `exit(AKGL_ERR_SDL)` is a wait status of 0 and the shell sees
 **`akgl_text_unloadallfonts()` before `SDL_Quit()`.** `SDL_Quit` destroys the property
 registry the fonts live in and takes the last reference to every one of them with it. This
 program loads no fonts, and the call is in the shutdown anyway because that is where it
-belongs the moment one is loaded — [Chapter 16](16-text-and-fonts.md).
+belongs the moment one is loaded — [Chapter 17](17-text-and-fonts.md).
 
 **Sixty `akgl_game_update` calls log `Low FPS! 0` sixty times.** `akgl_game.fps` is a
 completed-second average, so it reads 0 for the first second of the process, which is under
@@ -285,4 +285,4 @@ assertions in `tests/` work.
 
 [Chapter 5](05-the-heap.md) for the pools you just claimed a string from,
 [Chapter 7](07-the-game-and-the-frame.md) for what `akgl_game_update` actually does, or
-[Chapter 19](19-tutorial-sidescroller.md) to start building a game.
+[Chapter 20](20-tutorial-sidescroller.md) to start building a game.