fix: 非常量键成员 **= 复合赋值 rhs 生效

crates/oxide_emit/src/expr/assignment.rs

@@ -501,7 +501,9 @@ impl Emitter {
                 AssignmentOperator::Multiplication => OpCode::MUL,
                 AssignmentOperator::Division => OpCode::DIV,
                 AssignmentOperator::Remainder => OpCode::MOD,
-                AssignmentOperator::Exponential => OpCode::COMPOUND_EXP,
+                // 指数无 COMPOUND_* 对应（COMPOUND_EXP 只读 [Rd,A] 会忽略 rhs）：
+                // 走三寄存器 EXP，旧值在 a 槽（val_reg 兼 rd/a）、rhs 在 b 槽。
+                AssignmentOperator::Exponential => OpCode::EXP,
                 AssignmentOperator::BitwiseAnd => OpCode::BIT_AND,
                 AssignmentOperator::BitwiseOR => OpCode::BIT_OR,
                 AssignmentOperator::BitwiseXOR => OpCode::BIT_XOR,

crates/oxide_vm/tests/object_tests.rs

@@ -91,6 +91,24 @@ fn eval_computed_compound_assign_preserves_read_before_rhs() {
     assert_eq!(eval("var o={a:5}; o.a += 1, o.a"), "6");
 }
 
+#[test]
+fn eval_dynamic_key_compound_exp_uses_rhs() {
+    // 非常量键走 DYNAMIC 路径：`**=` 的指数取 RHS（EXP 读 [A,B]，旧值 a 槽、rhs b 槽）。
+    assert_eq!(eval("var o={a:5}; var k='a'; o[k]**=2, o.a"), "25");
+    assert_eq!(eval("var o={a:5}; var k='a'; o[k]**=2"), "25");
+    // 负 number 指数：5 ** -1 = 0.2。
+    assert_eq!(eval("var o={a:5}; var k='a'; o[k]**=-1, o.a"), "0.2");
+    // BigInt：结果保持 BigInt。
+    assert_eq!(eval("var o={a:5n}; var k='a'; o[k]**=3n, o.a"), "BigInt(125)");
+    // 负 BigInt 指数抛 RangeError；BigInt/Number 混合抛 TypeError。
+    let err = eval("var o={a:5n}; var k='a'; o[k]**=-1n");
+    assert!(err.contains("RangeError"), "expected RangeError, got: {err}");
+    let err2 = eval("var o={a:5}; var k='a'; o[k]**=2n");
+    assert!(err2.contains("TypeError"), "expected TypeError, got: {err2}");
+    // 求值序：先读旧值再求值 RHS——RHS 副作用改写属性不影响被求幂的旧值。
+    assert_eq!(eval("var o={a:5}; var k='a'; function f(){o.a=100; return 2;} o[k]**=f(), o.a"), "25");
+}
+
 #[test]
 fn eval_computed_compound_assign_getter_order() {
     // getter 场景：先触发 getter 读取，再求值 RHS。