Document the UI subsystem: chapter 22, with the appendix moving to 23

docs/22-ui.md covers the clay-backed UI: what clay owns against what libakgl
owns, bring-up and the arena refusal contract, the frame bracket and where it
sits in a real frame, the consumed-event rules (with the one-frame-stale hit
test stated plainly), the three widgets, menus across keyboard, gamepad and
mouse, writing raw CLAY() with an application-owned press edge, images and
styles, and how layout errors surface from frame_end with a captured trace.
Every listing is a checked block: one linked-and-run example with pinned
output, eight excerpts quoting examples/uidemo and examples/jrpg/textbox.c,
and the chapter figure is a frame out of uidemo via docs_game_figures.

The chapter owes and pays the comparison the tutorials earn: the dialog
widget's default style reproduces the JRPG textbox palette, the two listings
sit side by side, and the trade is stated -- 125 lines you own completely
against one call that costs you the subsystem. Neither is deprecated;
chapter 21 keeps teaching the hand-rolled panel on purpose.

The appendix moves to 23 (links in chapters 4, 5, 6, 7, 13 and the TOC
updated) and gains the ui.h status cross-reference, the AKGL_UI_* limits
table, and the new draw primitives' rows. Its status-band figures were stale
at COUNT 5 / LIMIT 261 while the band already held six codes; now correct at
7 / 263. Chapter 4 gains AKGL_ERR_UI (262) in the excerpt, the table and the
name list.

Co-Authored-By: Claude Code (Claude Fable 5, claude-fable-5) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

docs/README.md

@@ -34,7 +34,8 @@ per-function reference, build the Doxygen output with `doxygen Doxyfile`.
 | **[19. Utilities](19-utilities.md)** | Rectangle overlap, path resolution, the JSON accessors, static strings |
 | **[20. Tutorial: a 2D sidescroller](20-tutorial-sidescroller.md)** | Thirteen steps from an empty directory to a game with gravity, a jump, coins and hazards. **Start here** |
 | **[21. Tutorial: a top-down JRPG](21-tutorial-jrpg.md)** | Thirteen more, for four-way movement, NPCs, a text box, a follower, and freezing the world |
-| **[22. Appendix](22-appendix-limits.md)** | Every limit, every status, every configuration property |
+| **[22. User interfaces](22-ui.md)** | Menus, HUDs and dialogs on the clay layout engine: the widgets, the frame bracket, and writing `CLAY()` yourself |
+| **[23. Appendix](23-appendix-limits.md)** | Every limit, every status, every configuration property |
 
 **If you are new, read chapter 20 first and read it in order.** It builds a working game from
 an empty directory and teaches the library as it needs each piece; chapter 21 assumes it.