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/types.rs

@@ -2040,7 +2040,9 @@ pub fn unify(t1: &Type, t2: &Type) -> Result<Substitution, String> {
             // Function's required effects (e1) must be a subset of available effects (e2)
             // A pure function (empty effects) can be called anywhere
             // A function requiring {Logger} can be called in context with {Logger} or {Logger, Console}
-            if !e1.is_subset(&e2) {
+            // When expected effects (e2) are empty, it means "no constraint" (e.g., callback parameter)
+            // so we allow any actual effects through
+            if !e2.is_empty() && !e1.is_subset(&e2) {
                 return Err(format!(
                     "Effect mismatch: expected {{{}}}, got {{{}}}",
                     e1, e2