feat: 二元幂运算 opcode 支持 number 与 BigInt

- 新增 EXP opcode（0x94）与 emit 层 Exponential 分支，二元 a ** b 不再 compile error
- dispatch_exp 与 dispatch_compound_exp 共享 exp_result：IEEE-754 powf 快路径 +
  BigInt 幂（负/超 u32 指数 RangeError）+ 混合 BigInt/Number TypeError
- 复合 **= 补 BigInt 语义（此前走 f64 近似），与二元 ** 行为一致

crates/oxide_bytecode/src/opcode.rs

@@ -193,6 +193,10 @@ define_opcodes! {
     COMPOUND_EXP = 0x0B => "COMPOUND_EXP",
         def = Some(SlotSpec::Slot(Slot::Rd)), uses = [SlotSpec::Slot(Slot::Rd), SlotSpec::Slot(Slot::A)],
         pure = false, jump = false, term = false, ic = false,
+    // 二元幂运算 `a ** b`（三寄存器，与 SUB/MUL 同族；0x00-0x0F 已满，值用空闲槽 0x94）。
+    EXP = 0x94 => "EXP",
+        def = Some(SlotSpec::Slot(Slot::Rd)), uses = [SlotSpec::Slot(Slot::A), SlotSpec::Slot(Slot::B)],
+        pure = true, jump = false, term = false, ic = false,
     MOV = 0x0C => "MOV", // rd=dst, a=src（寄存器复制，区间拆分搬值）
         def = Some(SlotSpec::Slot(Slot::Rd)), uses = [SlotSpec::Slot(Slot::A)],
         pure = true, jump = false, term = false, ic = false,