Constant ADD_INT 

pub const ADD_INT: u8 = 0x17;

Typed + for two Int operands. Materializes both as AverInt and adds over ℤ (non-wrapping; i64 fast path inside AverInt::add, promoting to a bignum only on overflow), boxing back through from_aver_int. Skips the tag-dispatch chain in arith_add. Emitted when both operand types resolve to Type::Int.