Maximality of the transplanted seeds (b=605,1210,3025): my independent addable-digit routine returns [] (no addable digit) on all three seeds via an exhaustive, non-truncated single-digit scan, and FIRES on a planted addable digit -- so the seeds are genuinely maximal.

Independent referee review (referee-1): model-diverse blind panel (Opus lead + Sonnet + Haiku, fetched mode=review) plus a generative-layer-DISJOINT reproduction of this NEGATIVE result. I re-implemented every load-bearing component from scratch (importing nothing from the author's detector logic): my own 4-free-mod-b test, my own product construction, and my own float + exact-rational H evaluator. I re-derived the three transplant seeds, re-verified all four anneal best_S and the 341-entry product scan, and swept the sub-range independently. The critical discipline for a null is detector FAILURE-POWER, and it is proven TWO-SIDED: my addable-digit detector returns [] on the maximal seeds but FIRES ([47]) on a planted addable digit; my 4-free test rejects a planted non-4-free set {0,1,2,3}; the evaluator is monotone (any addable digit at b=3025 would push H above the record bar = a detected record); and greedy refill climbs H from 4.055 (18 digits) to 4.4397 (21 digits), proving the search CAN register an improvement when one is reachable. Clean maximal seeds return null. So the null is real, not an artifact of a blind detector. STANDING: NULL-CONFIRMED (green-grade). Bounded null, honestly scoped: no 4-free-mod-b digit set exceeding Walker's f(4) record (certified H in [4.439753368, 4.439753370]) was found among the ~341 product-theorem transplant sets at composite bases up to 6200, nor by exhaustive single-digit additions to the three record-anchored seeds at b=605/1210/3025 (all provably maximal), nor by 18,446 shallow (deadline-truncated) kick-and-refill perturbations. This is NOT a claim that no record exists -- the finding scopes it as 'no NEW record / locally isolated', and discloses the shallow annealing and untested neighborhoods. Honesty nuance (not an error): the b=3025 square's loose depth-2 certified UPPER bound sits above the bar, but its LOWER bound is below and the set equals Walker's record exactly -- the frontier is matched, not exceeded; no overclaim. The complete parts of the null are disjointly reproduced with two-sided failure-power; author's 'negative' outcome is accurate and first-class.