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.

tools/test_gen_api.py

@@ -3,7 +3,7 @@
 import tempfile
 from pathlib import Path
 
-from gen_api import ROOT, parse
+from gen_api import ROOT, parse, render
 
 
 with tempfile.TemporaryDirectory(dir=ROOT) as directory:
@@ -30,4 +30,28 @@ doc = module["functions"][0][1]
 assert doc["doc"] == ["Reads one complete file."]
 assert doc["params"][0][2] == "Absolute file path."
 assert doc["returns"][0][1] == "File contents."
+
+# Callbacks render as fields of a class an app composes, never as global functions.
+with tempfile.TemporaryDirectory(dir=ROOT) as directory:
+    source = Path(directory) / "handlers.cpp"
+    source.write_text(
+        """// @lua-app TouchHandlers
+// @lua-field home? string The route back lands on.
+// ---Fired when the finger lands.
+// @param x integer
+// @param y integer
+// @lua-fn onTouchDown?
+// ---Mounts the route.
+// @param route string
+// @param arg string|nil
+// @lua-fn start
+"""
+    )
+    output = render(source, parse(source))
+
+assert "---@class TouchHandlers" in output
+assert "---@field home? string The route back lands on." in output
+assert "---@field onTouchDown? fun(x: integer, y: integer) Fired when the finger lands." in output
+assert "---@field start fun(route: string, arg?: string) Mounts the route." in output
+assert "function " not in output
 print("ok")