feat: add no-std common runtime

Add an alloc-backed common library with direct platform I/O.

Build a host attribute macro for async entry points and keep runtime
boilerplate outside application code. Compile the library, macro, and
example directly with rustc through CMake.

Co-Authored-By: luna (openrouter/openai/gpt-5.6-luna): runtime work

runtime/main.rs

@@ -0,0 +1,11 @@
+#![no_std]
+#![no_main]
+
+fn print(message: &str) {
+    let _ = rslibc::io::write_str(message);
+}
+
+#[rslibc_macros::main]
+async fn main() {
+    print("Hello\n");
+}