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/touch.lua

@@ -28,22 +28,12 @@ function touch.isTouched() end
 ---@return string? error
 function touch.setCalibration(x0, y0, x1, y1) end
 
----Fired when the finger lands.
----@param x integer
----@param y integer
-function on_touch_down(x, y) end
+-- What an app implements to see raw touch, composed into its own class:
+--
+--   ---@class PaintApp : App, TouchHandlers
 
----Fired when the finger moves while down, after the firmware's jitter filter.
----@param x integer
----@param y integer
-function on_touch_move(x, y) end
-
----Fired when the finger lifts.
----@param x integer
----@param y integer
-function on_touch_up(x, y) end
-
----Tap alias, fired on release like a click, after on_touch_up.
----@param x integer
----@param y integer
-function on_touch(x, y) end
+---@class TouchHandlers
+---@field onTouchDown? fun(x: integer, y: integer) Fired when the finger lands.
+---@field onTouchMove? fun(x: integer, y: integer) Fired when the finger moves while down, after the firmware's jitter filter.
+---@field onTouchUp? fun(x: integer, y: integer) Fired when the finger lifts.
+---@field onTouch? fun(x: integer, y: integer) Tap alias, fired on release like a click, after onTouchUp.