Explain objc_msgSend fast path in ARM64 in detail

objc_msgSend uses cache lookup → hash bucket → direct branch to IMP via registers.

On ARM64, objc_msgSend uses x0=receiver, x1=selector. It computes cache bucket index via selector hash. If hit, loads IMP into x17 and branches (br x17). No stack frame is created. On miss, slow path triggers method resolution and forwarding.