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>

README.md

@@ -17,8 +17,8 @@ The two tutorials build real, running programs that live under [`examples/`](exa
 
 | | |
 |---|---|
-| [A 2D sidescroller](docs/19-tutorial-sidescroller.md) | Gravity, a jump, collision you write yourself, coins and hazards |
-| [A top-down JRPG](docs/20-tutorial-jrpg.md) | A town map, NPCs spawned from map objects, four-way animation, a text box, a follower |
+| [A 2D sidescroller](docs/20-tutorial-sidescroller.md) | Gravity, a jump, collision you write yourself, coins and hazards |
+| [A top-down JRPG](docs/21-tutorial-jrpg.md) | A town map, NPCs spawned from map objects, four-way animation, a text box, a follower |
 
 For per-function reference, build the Doxygen output with `doxygen Doxyfile`; it is a CI
 gate, so an undocumented symbol fails the build.