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/test/runtime_test.cpp

@@ -200,11 +200,11 @@ int main() {
         "route .. ':' .. arg end,\n"
         "  draw = function(delta) if delta < 0 then error('kaboom') end\n"
         "    events[#events + 1] = 'draw:' .. delta end,\n"
-        "  on_touch_down = note('down'),\n"
-        "  on_touch_up = note('up'),\n"
-        "  on_touch = note('tap'),\n"
-        "  on_button_up = note('bup'),\n"
-        "  on_button = note('btap'),\n"
+        "  onTouchDown = note('down'),\n"
+        "  onTouchUp = note('up'),\n"
+        "  onTouch = note('tap'),\n"
+        "  onButtonUp = note('bup'),\n"
+        "  onButton = note('btap'),\n"
         "}\n";
     esp32lua::Runtime hosted(chrome.providers());
     assert(hosted.startApp("Reader", "book.epub"));
@@ -212,7 +212,7 @@ int main() {
     hosted.callDraw(33);
     hosted.callTouch(esp32lua::TouchPhase::Down, 5, 6);
     hosted.callTouch(esp32lua::TouchPhase::Move, 5,
-                     7); // main.lua defines no on_touch_move
+                     7); // main.lua defines no onTouchMove
     hosted.callTouch(esp32lua::TouchPhase::Up, 5, 8);
     hosted.callButton("confirm", false);
     run(hosted.state(),