Refactor native function registration and instantiation

Introduces `register_native` for direct registration of `NativeFunction`
and `register_native_fn` for convenience from closures. The
`Environment::run`
method is removed, and its functionality is now handled by
`Environment::instantiate`,
which packages the linked AST into an invokable `NativeFunction`. This
streamlines
the execution path and better separates compilation/linking from runtime
execution.

src/integration_test.rs

@@ -71,10 +71,11 @@ mod tests {
         let env = Environment::new();
         let compiled = env.compile(source).expect("Failed to compile");
         let linked = env.link(compiled);
-        let result = env.run(&linked);
+        let func = env.instantiate(linked);
+        let result: Result<Value, String> = Ok((func.func)(vec![]));
 
         match result {
-            Ok(Value::Int(val)) => assert_eq!(val, 20, "Closure should modify outer variable"),
+            Ok(Value::Int(20)) => (),
             Ok(val) => panic!("Expected Int(20), got {:?}", val),
             Err(e) => panic!("VM Error: {}", e),
         }