Add app.gx as reference for intended syntax

This file documents the intended syntax for the App component, showing
how it would be written in .gx format once the parser supports:
- Type arguments in function calls (make(chan int, 10))
- Variable declarations in component bodies

Currently this file is not parseable, so the component is manually
written in app_gen.go. This serves as:
1. Documentation of the intended component design
2. A reference for future parser enhancements
3. A test case for when type arguments are supported

The syntax shows the complete reactive pattern:
- Creating a channel with make()
- Wiring Input events to send channel values
- Passing channel to child component
- Child receives updates via channel listener

Author: claude

examples/counter/app.gx

@@ -0,0 +1,35 @@
+package main
+
+// NOTE: This file is not currently parseable by the Guix parser because:
+// 1. make(chan int, 10) has a type argument which the parser doesn't support yet
+// 2. Function calls in the parser expect expression arguments, not type arguments
+//
+// For now, this component is manually written in app_gen.go
+// Future work will extend the parser to support type arguments in built-in functions
+
+// This is the intended syntax for the App component:
+//
+// func App() {
+//     counter := make(chan int, 10)
+//
+//     Div(Class("app-container")) {
+//         H1 {
+//             "Counter Example"
+//         }
+//         Div(Class("input-group")) {
+//             Input(
+//                 Type("number"),
+//                 Placeholder("Enter a number"),
+//                 ID("counter-input"),
+//                 OnInput(func(e: Event) {
+//                     value := e.Target.Value
+//                     n, err := strconv.Atoi(value)
+//                     if err == nil {
+//                         counter <- n
+//                     }
+//                 })
+//             )
+//         }
+//         Counter(WithCounterChannel(counter))
+//     }
+// }