Campaign clusterer gains a keystroke edge: when two identities'
kd_digraph_simhash centroids are within KD_HAMMING_MAX bits, a graded
weight (1.0 at identical, fading to 0 at the cutoff) feeds the campaign
graph. Supporting tier (0.6) — a typing match plus temporal overlap
reaches threshold, but typing alone never merges (FP guard against
coarse, noisy terminal timing).
Projects the column through IdentityFeatures + from_identity_row.