Stryker survivors 12390 (initial _columns +1 skips the first resize at n = 1) and
12399/12400 (A not cleared between same-count calls) get tests. Array.Clear on b was an
equivalent mutant: rows 0-6 are assigned every call and the ridge rows are never written.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>