refactor: Use declarative component composition for webgpu-cube

Replace manual component wrapper (app_custom.go) with declarative Guix
component composition pattern. The app.gx now properly includes the
Controls component using the same pattern as the counter example.

Changes:
- Update app.gx to declaratively include Controls(WithCommands(commands))
- Create command channel using make() directly (hoisted by codegen)
- Remove app_custom.go wrapper entirely
- Add minimal app_helpers.go with StartCommandProcessor() method
- Update main.go to use generated NewApp() directly
- Regenerate app_gen.go with proper component composition

The generated code now:
- Hoists the commands channel to a struct field
- Initializes Controls component in NewApp()
- Calls BindApp() on the Controls instance
- Renders Controls inline via c.controlsInstance.Render()

This follows the Guix design principle that component composition
should be handled by the code generator, not manual Go wrappers.

Author: claude

examples/webgpu-cube/app.gx

@@ -1,9 +1,12 @@
 package main
 
 func App() (Component) {
+	commands := make(chan ControlCommand, 10)
+
 	Div(
 		Class("webgpu-container"),
-		TabIndex(0)
+		TabIndex(0),
+		OnKeyDown(makeKeyboardHandler(commands))
 	) {
 		Canvas(
 			ID("webgpu-canvas"),
@@ -12,6 +15,7 @@ func App() (Component) {
 		) {
 			GPUScene(NewCubeScene(0, 0))
 		}
+		Controls(WithCommands(commands))
 		Div(Class("loading")) {
 			"Loading WebGPU..."
 		}

examples/webgpu-cube/app_custom.go

@@ -0,0 +1,12 @@
+//go:build js && wasm
+
+package main
+
+// StartCommandProcessor starts the goroutine that processes control commands
+func (a *App) StartCommandProcessor() {
+	go func() {
+		for cmd := range a.commands {
+			processControlCommand(cmd)
+		}
+	}()
+}

examples/webgpu-cube/app_gen.go

@@ -22,26 +22,34 @@ func main() {
 	runtime.WaitForDOMReady(func() {
 		log("[Go] DOM is ready, initializing app")
 
-		// Create and mount app component with controls
-		log("[Go] Creating AppWithControls...")
-		appComponent := NewAppWithControls()
-		log("[Go] AppWithControls created")
+		// Create app using generated component
+		log("[Go] Creating App...")
+		appComponent := NewApp()
+		log("[Go] App created")
 
+		// Start command processor goroutine
+		log("[Go] Starting command processor...")
+		appComponent.StartCommandProcessor()
+		log("[Go] Command processor started")
+
+		// Create runtime app
 		log("[Go] Creating runtime app...")
 		runtimeApp := runtime.NewApp(appComponent)
 		log("[Go] Runtime app created")
 
+		// Bind app
 		log("[Go] Binding app...")
 		appComponent.BindApp(runtimeApp)
 		log("[Go] App bound")
 
+		// Mount
 		log("[Go] Mounting to #app...")
 		if err := runtimeApp.Mount("#app"); err != nil {
 			log("[Go] Mount error:", err)
 			panic(err)
 		}
 
-		log("[Go] App mounted successfully with integrated controls")
+		log("[Go] App mounted successfully")
 	})
 
 	// Keep the program running