fix: add stub main() for non-WASM builds in cmd/wasm

The WASM main.go is gated behind //go:build js && wasm, leaving
register.go (//go:build !js) as the only file on native builds —
but it had no main(). Adds a stub that prints a usage hint.

Co-Authored-By: Virgil <virgil@lethean.io>

cmd/wasm/register.go

@@ -20,3 +20,7 @@ func buildComponentJS(slotsJSON string) (string, error) {
 	}
 	return codegen.GenerateBundle(slots)
 }
+
+func main() {
+	fmt.Println("go-html WASM module — build with GOOS=js GOARCH=wasm")
+}