feat: 添加三元运算符支持，更新解析器和解释器以处理条件表达式

src/parser/parser.rs

@@ -208,8 +208,24 @@ impl Parser {
         self.assignment()
     }
 
-    fn assignment(&mut self) -> Result<Expr, RuntimeError> {
+    fn ternary(&mut self) -> Result<Expr, RuntimeError> {
         let expr = self.logical_or()?;
+        if self.match_kind(&[TokenKind::Question]) {
+            let then_branch = self.assignment()?;
+            self.consume(TokenKind::Colon, "Expected ':' after then branch of ternary.")?;
+            let else_branch = self.assignment()?;
+            Ok(Expr::Ternary {
+                condition: Box::new(expr),
+                then_branch: Box::new(then_branch),
+                else_branch: Box::new(else_branch),
+            })
+        } else {
+            Ok(expr)
+        }
+    }
+
+    fn assignment(&mut self) -> Result<Expr, RuntimeError> {
+        let expr = self.ternary()?;
 
         let op = if self.match_kind(&[TokenKind::Equal]) {
             Some(AssignOp::Equal)
@@ -814,6 +830,44 @@ mod tests {
             }
         }
 
+        // ----- ternary -----
+
+        #[test]
+        fn parse_ternary_simple() {
+            match parse_expr("a ? 1 : 2") {
+                Expr::Ternary { condition, then_branch, else_branch } => {
+                    assert!(matches!(*condition, Expr::Variable(_)));
+                    assert!(matches!(*then_branch, Expr::Literal(Literal::Number(1.0))));
+                    assert!(matches!(*else_branch, Expr::Literal(Literal::Number(2.0))));
+                }
+                e => panic!("Expected Ternary, got {:?}", e),
+            }
+        }
+
+        #[test]
+        fn parse_ternary_nested_right_assoc() {
+            // a ? b : c ? d : e  →  a ? b : (c ? d : e)
+            match parse_expr("a ? b : c ? d : e") {
+                Expr::Ternary { condition, then_branch, else_branch } => {
+                    assert!(matches!(*condition, Expr::Variable(_)));
+                    assert!(matches!(*then_branch, Expr::Variable(_)));
+                    assert!(matches!(*else_branch, Expr::Ternary { .. }));
+                }
+                e => panic!("Expected Ternary with nested Ternary else, got {:?}", e),
+            }
+        }
+
+        #[test]
+        fn parse_ternary_with_assignment_in_else() {
+            // a ? b : c = d  →  a ? b : (c = d)
+            match parse_expr("a ? b : c = 5") {
+                Expr::Ternary { else_branch, .. } => {
+                    assert!(matches!(*else_branch, Expr::Assign { .. }));
+                }
+                e => panic!("Expected Ternary with Assign else, got {:?}", e),
+            }
+        }
+
         // ----- assignment -----
 
         #[test]