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.

native/src/bindings/features/buttons.cpp

@@ -72,13 +72,17 @@ void registerButtons(lua_State* state) {
 } // namespace bindings
 } // namespace esp32lua
 
-// @lua-global
+// @lua-app ButtonHandlers
+// @lua-preamble -- What an app implements to see buttons, composed into its own
+// class:
+// @lua-preamble --
+// @lua-preamble --   ---@class MenuApp : App, ButtonHandlers
 // ---Fired when a button goes down.
 // @param button Button
-// @lua-fn on_button_down
+// @lua-fn onButtonDown?
 // ---Fired when a button comes up.
 // @param button Button
-// @lua-fn on_button_up
-// ---Tap alias, fired on release like a click, after on_button_up.
+// @lua-fn onButtonUp?
+// ---Tap alias, fired on release like a click, after onButtonUp.
 // @param button Button
-// @lua-fn on_button
+// @lua-fn onButton?