Fill the gaps the chapter 22 weaker-model reader found

The AGENTS.md "Testing a Tutorial" pass: chapter 22 was handed to a reader on
a weaker model with the library tree stripped of examples/ and docs/, and
they had to build a menu/HUD/dialog program from the chapter alone. They
built one that compiled and ran -- and reported success their own screenshots
disproved, which is why the protocol says to verify: their synthesized key
events set only the scancode, akgl_ui_menu_handle_event matches keycodes, and
their program never actually left the title screen. With that one line fixed,
everything they had built from the chapter worked.

Three findings survived verification against eight reported:

- The chapter never stated its prerequisites, so the reader burned most of
  the session on library bring-up it does not cover and invented goto error
  handling for main(). A new "What this chapter assumes" paragraph points at
  chapters 3, 7 and 17, and at the demo's startup() and main() as the
  complete reference.
- Nothing named the menu's exact keys, or that they are keycodes rather than
  scancodes -- invisible with a real keyboard, fatal for synthesized events,
  which the demo's own --demo script teaches. The menus section now names
  SDLK_UP/DOWN/RETURN, the gamepad buttons, and the key.key trap.
- Verifying their dialog screenshot showed a message wrapping past the fixed
  AKGL_UI_DIALOG_HEIGHT runs visibly out of the panel. Deliberate -- same as
  the hand-rolled textbox, and visible overflow beats silent truncation --
  but undocumented; akgl_ui_dialog's header note now says so.

Rejected for the record, per protocol: akgl_game_init already initializes the
property registry (src/game.c:219), and the font they "found" in a different
directory was the build tree's file(COPY) of tests/assets.

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

include/akgl/ui.h

@@ -391,6 +391,13 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_ui_frame_end(akgl_RenderBackend *self);
  *              -- clay keeps the pointer, not a copy. May be empty, which
  *              draws an empty panel.
  * @param style Look to draw with, or `NULL` for the textbox palette.
+ *
+ * @note The panel's height is fixed at #AKGL_UI_DIALOG_HEIGHT and the text is
+ *       not clipped to it: a message that wraps to more lines than fit runs
+ *       past the bottom border, visibly. That is the same behaviour as the
+ *       hand-rolled panel this widget mirrors, and it is deliberate --
+ *       overflow you can see beats truncation you cannot. Size the height for
+ *       your longest message, or raise the override.
  * @return `NULL` on success, otherwise an error context owned by the caller.
  * @throws AKERR_NULLPOINTER If @p id or @p text is `NULL`.
  * @throws AKGL_ERR_UI If the subsystem is not initialized or no frame is