Document collision, and correct what it made false elsewhere

The new chapter 15 covers the opt-in setup, where resolution runs in the step
and why it runs after the move, shapes and the z-extrusion trap, the asymmetric
masks and the defaults that matter more than the mechanism, tiles as a source
rather than proxies, the response hook and the three ways it is easy to write
wrongly, sensors, the four queries, both partitioners, and what is still not
implemented.

Collision falsified claims in eight other chapters. The physics chapter said
"There is no collision detection, at all" and that collide always raises
AKERR_API; actors had six behaviour hooks; the heap had five pools and four
non-claiming acquires; the status band held five codes; and the design
philosophy had exactly two pluggable subsystems. The appendix gains a
collision.h status section, the three new pool ceilings and a table of the
collision constants that are deliberately not in a public header.

Also fixes chapter labels the renumbering commit left pointing at their old
numbers -- "18. Utilities" linked to 19-utilities.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

docs/07-the-game-and-the-frame.md

@@ -88,7 +88,7 @@ Verified against `src/game.c`. The order matters in three places, marked:
 5. Create the state mutex. Failure is `AKGL_ERR_SDL`.
 6. **Take the state lock.** Everything from here to the end runs holding it.
 7. Check `name`, `version` and `uri` are non-empty.
-8. `akgl_heap_init()` — zero all five pools.
+8. `akgl_heap_init()` — zero all eight pools.
 9. The eight registry initializers: actor, sprite, spritesheet, character, font, music,
    properties, actor-state-strings. **Note that `akgl_registry_init` is not called** —
    `akgl_game_init` calls the eight individually. See [Chapter 6](06-the-registry.md).