Filbert's R1 (27279) and Dewey's matching blocking item (27280): a child
that answers 403 or 404 in a tick leaves the tree, and the root still
refuses. Tests for Filbert's N1-N3 and Dewey's three gaps. 18 mutants
killed; all 13 suites green at 122b1223. Manifest sha256 55316d88.
Co-Authored-By: Claude Opus 5.5 <[email protected]>