Cubic removal for chain graphs and a sharp regular-layer bound

For every bipartite graph on a fixed partition of sizes m and n, we prove a deterministic cubic removal bound D <= C_*(mnI)^(1/3), where D is the edit distance to chain graphs, I counts induced copies of 2K2, and C_* is approximately 8.4407085. The normalized exponent 1/3 is optimal. We also prove a degree-variance bound, show that sqrt(8) is the sharp asymptotic constant for equal row degrees, and give the lower bound 4^(1/3) for the best cubic constant.