Normalize DNL/INL code axis

[chenzc24]

Summary

• Normalize DNL/INL x-axis to 0..1 when num_bits is known, with label Code (2^N LSB).
• Keep the raw code axis when num_bits is omitted.
• Rename the exp_a32 output image to match the script number.

Tests

• uv run --with pytest pytest tests/unit/aout/test_inl_from_ramp.py tests/unit/aout/test_inl_from_sine_sweep_length.py -q
• uv run python src/adctoolbox/examples/04_debug_analog/exp_a32_inl_from_sine_sweep_length.py