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/12-actors.md

@@ -295,7 +295,7 @@ The `akgl_actor_cmhf_*` functions ("control map handler function") are what
 `akgl_controller_default` binds the arrow keys and the D-pad to, and what a game's
 own `akgl_Control` bindings can point at. They take the control map's **target
 actor**, not a global "player", so they work for any number of locally controlled
-actors. See [Chapter 15](15-input.md).
+actors. See [Chapter 16](16-input.md).
 
 | Handler | Sets | Clears first | Writes |
 |---|---|---|---|
@@ -441,7 +441,7 @@ for `akgl_Actor`.
 
 - [Chapter 11](11-characters.md) — the template, and the state-to-sprite map.
 - [Chapter 14](14-physics.md) — what happens to `t`, `e`, `v` and `a` each step.
-- [Chapter 15](15-input.md) — control maps, and binding the handlers above.
+- [Chapter 16](16-input.md) — control maps, and binding the handlers above.
 - [Chapter 08](08-rendering.md) — where `renderfunc` gets called from.
 - [Chapter 04](04-errors.md) — `AKGL_ERR_LOGICINTERRUPT` and the rest of the
   status tables.