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/ast/rtl/datetime.rs

@@ -8,7 +8,7 @@ pub fn register(env: &Environment) {
         ret: StaticType::DateTime,
     }));
 
-    env.register_native("date", date_ty, Purity::Pure, |args| {
+    env.register_native_fn("date", date_ty, Purity::Pure, |args| {
         if let Value::Text(s) = &args[0] {
             // Try parse YYYY-MM-DD
             if let Ok(dt) = NaiveDate::parse_from_str(s, "%Y-%m-%d") {
@@ -32,7 +32,7 @@ pub fn register(env: &Environment) {
         ret: StaticType::DateTime,
     }));
 
-    env.register_native("now", now_ty, Purity::SideEffectFree, |_| {
+    env.register_native_fn("now", now_ty, Purity::SideEffectFree, |_| {
         let ts = chrono::Utc::now().timestamp_millis();
         Value::DateTime(ts)
     });