Expand documentation on selftest.sh behavior, module matching, and kernel compatibility. Clarify the auto-selection process based on `vermagic` and provide examples to guide users through fallback handling and manual overrides.
Replace legacy gpio_to_desc() with GPIO descriptor resolution by chip label
and hardware number, ensuring compatibility with kernels using non-zero
gpiochip bases. Added `selftest.sh` for non-persistent module testing, which
performs a hardware self-test and verifies wiring before connecting the real
bus. Included a detailed README.md documenting the self-test process.
