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/core/runtime.lua

@@ -4,20 +4,17 @@
 
 -- The firmware loads /.lua/main.lua into every fresh state and calls these on the
 -- table it returns. Where apps live, what surrounds them and which of these an app
--- itself sees are all main.lua's to decide.
+-- itself sees are all main.lua's to decide, which is why an app composes the
+-- classes for the features it handles:
 --
--- Fields the firmware reads: home, the route sys.back() lands on once history is
--- empty, and data, the sys.getAppDataPath() template whose ? is the app id.
+--   ---@class PaintApp : App, TouchHandlers
 --
 -- The firmware does not clear the frame before calling draw(), and commits changed
 -- display content after each callback batch using the panel's own refresh policy.
 -- Timer callbacks are registered directly with timer.after/every.
 
----Required. Mounts the route; failing here leaves no app running.
----@param route string The app path sys.launch, sys.back or the boot recorded.
----@param arg? string The string passed to sys.launch or sys.replace.
-function start(route, arg) end
-
----Optional frame loop, called once after start and then at most 30 FPS, best effort.
----@param deltaMs integer Monotonic milliseconds since the previous draw; zero on the first.
-function draw(deltaMs) end
+---@class App
+---@field home? string The route sys.back() lands on once history is empty.
+---@field data? string The sys.getAppDataPath() template whose ? is the app id.
+---@field start fun(route: string, arg?: string) Required. Mounts the route; failing here leaves no app running.
+---@field draw? fun(deltaMs: integer) Optional frame loop, called once after start and then at most 30 FPS, best effort.