feat: add tuple index access, multiline args, and effect unification fix

- Tuple index: `pair.0`, `pair.1` syntax across parser, typechecker,
  interpreter, C/JS backends, formatter, linter, and symbol table
- Multi-line function args: allow newlines inside argument lists
- Fix effect unification for callback parameters (empty expected
  effects means "no constraint", not "must be pure")

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -4831,6 +4831,71 @@ c")"#;
         }
     }
 
+    // ============ Multi-line Arguments Tests ============
+
+    #[test]
+    fn test_multiline_function_args() {
+        let source = r#"
+            fn add(a: Int, b: Int): Int = a + b
+            let result = add(
+                1,
+                2
+            )
+        "#;
+        assert_eq!(eval(source).unwrap(), "3");
+    }
+
+    #[test]
+    fn test_multiline_function_args_with_lambda() {
+        let source = r#"
+            let xs = List.map(
+                [1, 2, 3],
+                fn(x) => x * 2
+            )
+        "#;
+        assert_eq!(eval(source).unwrap(), "[2, 4, 6]");
+    }
+
+    // ============ Tuple Index Tests ============
+
+    #[test]
+    fn test_tuple_index_access() {
+        let source = r#"
+            let pair = (42, "hello")
+            let first = pair.0
+        "#;
+        assert_eq!(eval(source).unwrap(), "42");
+    }
+
+    #[test]
+    fn test_tuple_index_access_second() {
+        let source = r#"
+            let pair = (42, "hello")
+            let second = pair.1
+        "#;
+        assert_eq!(eval(source).unwrap(), "\"hello\"");
+    }
+
+    #[test]
+    fn test_tuple_index_triple() {
+        let source = r#"
+            let triple = (1, 2, 3)
+            let sum = triple.0 + triple.1 + triple.2
+        "#;
+        assert_eq!(eval(source).unwrap(), "6");
+    }
+
+    #[test]
+    fn test_tuple_index_in_function() {
+        let source = r#"
+            fn first(pair: (Int, String)): Int = pair.0
+            fn second(pair: (Int, String)): String = pair.1
+            let p = (42, "hello")
+            let result = first(p)
+        "#;
+        assert_eq!(eval(source).unwrap(), "42");
+    }
+
     // Exhaustiveness checking tests
     mod exhaustiveness_tests {
         use super::*;