Lal and cubibobi, thank you both for your considered responses, which get to the heart of my question.
If I understand, then, what the third and fourth bases of anapanasati refer to by kāyasaṅkhāraṁ is neither “the body of the breath (as experienced at the tip of the nose)” nor “the breath as it occurs within the whole body (of the meditator)” – but rather to sankhara (as used in the Abhidhamma) of the body? As we progress through stages of anapanasati compounded ideas of the physical body are tranquilised?
– We may not even realize it. But engaging in “kaya sankhara” (especially immoral ones) brings stress to the mind. – This seems like sila more than samadhi to me. Any further explanation that might help me to clarify my view is appreciated.