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/04-errors.md

@@ -97,7 +97,7 @@ Statuses raised by the libraries underneath also reach you unchanged. `aksl_fope
 `akgl_error_init` can raise libakerror's `AKERR_STATUS_RANGE_OVERLAP`,
 `AKERR_STATUS_RANGE_FULL` or `AKERR_STATUS_NAME_FULL`.
 
-[Chapter 21](21-appendix-limits.md) has the per-function cross-reference.
+[Chapter 22](22-appendix-limits.md) has the per-function cross-reference.
 
 ## Table 3 — the exit status trap
 
@@ -264,6 +264,6 @@ Four things in there are libakgl-specific and worth naming:
   release.
 - [Chapter 6](06-the-registry.md) — the name lookups behind `AKERR_KEY` and
   `AKERR_NULLPOINTER`.
-- [Chapter 21](21-appendix-limits.md) — which function raises what.
+- [Chapter 22](22-appendix-limits.md) — which function raises what.
 - `deps/libakerror/README.md` — the protocol itself, and the exit-status discussion.
 - The generated Doxygen reference — every function's own `@throws` list.