find the persistent pair record for a body pair by walking bodyA's pair list — O(pair degree). returns the record index, or -1 if no record exists. (discovery dedup and the CCD find-or-create path both use this.)
find the persistent pair record for a body pair by walking bodyA's pair list — O(pair degree). returns the record index, or -1 if no record exists. (discovery dedup and the CCD find-or-create path both use this.)