169. Instruction bmi

Why this step exists

"Minus" means the Negative flag from an earlier result is set, so BMI consumes that status without recomputing it.

In this step, add only this implementation

`emulator/cpu/instructions.py::bmi`::

def bmi(cpu: CPU, offset: int):
    if cpu.flags.get_negative_flag():
        cpu.pc = (cpu.pc + offset) & 0xFFFF

BMI applies the already decoded signed displacement to the PC after its operand, masking the target to 16 bits.

Invariants: Negative set takes the branch and Negative clear leaves PC alone; flags, other registers, and memory are unchanged. Misconception: BMI does not inspect the sign of `offset` or perform arithmetic to establish Negative.

Out of scope: BVC/BVS are lessons 170-171. Imports into `emulator/cpu/opcodes.py`, relative handlers, and opcode table entries belong to lessons 172-179.