feat: add List.findIndex, List.zip, List.flatten, List.contains

Add missing List operations requested by ergon game engine project:
- findIndex(list, predicate) -> Option<Int>
- zip(list1, list2) -> List<(A, B)>
- flatten(listOfLists) -> List<A>
- contains(list, element) -> Bool

Resolves ergon porting blocker #4.

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

src/interpreter.rs

@@ -83,10 +83,14 @@ pub enum BuiltinFn {
     // Additional List operations
     ListIsEmpty,
     ListFind,
+    ListFindIndex,
     ListAny,
     ListAll,
     ListTake,
     ListDrop,
+    ListZip,
+    ListFlatten,
+    ListContains,
 
     // Additional String operations
     StringStartsWith,
@@ -974,10 +978,14 @@ impl Interpreter {
                 Value::Builtin(BuiltinFn::ListIsEmpty),
             ),
             ("find".to_string(), Value::Builtin(BuiltinFn::ListFind)),
+            ("findIndex".to_string(), Value::Builtin(BuiltinFn::ListFindIndex)),
             ("any".to_string(), Value::Builtin(BuiltinFn::ListAny)),
             ("all".to_string(), Value::Builtin(BuiltinFn::ListAll)),
             ("take".to_string(), Value::Builtin(BuiltinFn::ListTake)),
             ("drop".to_string(), Value::Builtin(BuiltinFn::ListDrop)),
+            ("zip".to_string(), Value::Builtin(BuiltinFn::ListZip)),
+            ("flatten".to_string(), Value::Builtin(BuiltinFn::ListFlatten)),
+            ("contains".to_string(), Value::Builtin(BuiltinFn::ListContains)),
             (
                 "forEach".to_string(),
                 Value::Builtin(BuiltinFn::ListForEach),
@@ -2723,6 +2731,55 @@ impl Interpreter {
                 Ok(EvalResult::Value(Value::Bool(true)))
             }
 
+            BuiltinFn::ListFindIndex => {
+                let (list, func) = Self::expect_args_2::<Vec<Value>, Value>(&args, "List.findIndex", span)?;
+                for (i, item) in list.iter().enumerate() {
+                    let v = self.eval_call_to_value(func.clone(), vec![item.clone()], span)?;
+                    match v {
+                        Value::Bool(true) => {
+                            return Ok(EvalResult::Value(Value::Constructor {
+                                name: "Some".to_string(),
+                                fields: vec![Value::Int(i as i64)],
+                            }));
+                        }
+                        Value::Bool(false) => {}
+                        _ => return Err(err("List.findIndex predicate must return Bool")),
+                    }
+                }
+                Ok(EvalResult::Value(Value::Constructor {
+                    name: "None".to_string(),
+                    fields: vec![],
+                }))
+            }
+
+            BuiltinFn::ListZip => {
+                let (list1, list2) = Self::expect_args_2::<Vec<Value>, Vec<Value>>(&args, "List.zip", span)?;
+                let result: Vec<Value> = list1
+                    .into_iter()
+                    .zip(list2.into_iter())
+                    .map(|(a, b)| Value::Tuple(vec![a, b]))
+                    .collect();
+                Ok(EvalResult::Value(Value::List(result)))
+            }
+
+            BuiltinFn::ListFlatten => {
+                let list = Self::expect_arg_1::<Vec<Value>>(&args, "List.flatten", span)?;
+                let mut result = Vec::new();
+                for item in list {
+                    match item {
+                        Value::List(inner) => result.extend(inner),
+                        other => result.push(other),
+                    }
+                }
+                Ok(EvalResult::Value(Value::List(result)))
+            }
+
+            BuiltinFn::ListContains => {
+                let (list, target) = Self::expect_args_2::<Vec<Value>, Value>(&args, "List.contains", span)?;
+                let found = list.iter().any(|item| Value::values_equal(item, &target));
+                Ok(EvalResult::Value(Value::Bool(found)))
+            }
+
             BuiltinFn::ListTake => {
                 let (list, n) = Self::expect_args_2::<Vec<Value>, i64>(&args, "List.take", span)?;
                 let n = n.max(0) as usize;