refactor(api)!: declare callbacks as classes an app composes

The runtime has always called fields on the table main.lua returns, but
@lua-global declared them as loose functions, so the stubs type-checked
something that does not exist and read as "define a global".

Callbacks are now @lua-app blocks that generate a class: App for the core
contract, TouchHandlers and ButtonHandlers beside the namespaces they belong
to. An app composes what it implements:

  ---@class PaintApp : App, TouchHandlers

Names follow the rest of the surface: onTouchDown rather than on_touch_down,
with the field names the runtime looks up renamed to match. @lua-field carries
the plain fields (home, data) that were prose in a preamble before.

lua/api/features/buttons.lua

@@ -33,14 +33,11 @@ function buttons.wasPressed(button) end
 ---@return boolean
 function buttons.wasReleased(button) end
 
----Fired when a button goes down.
----@param button Button
-function on_button_down(button) end
+-- What an app implements to see buttons, composed into its own class:
+--
+--   ---@class MenuApp : App, ButtonHandlers
 
----Fired when a button comes up.
----@param button Button
-function on_button_up(button) end
-
----Tap alias, fired on release like a click, after on_button_up.
----@param button Button
-function on_button(button) end
+---@class ButtonHandlers
+---@field onButtonDown? fun(button: Button) Fired when a button goes down.
+---@field onButtonUp? fun(button: Button) Fired when a button comes up.
+---@field onButton? fun(button: Button) Tap alias, fired on release like a click, after onButtonUp.