H5 anchor (mechanism decider, status=control, NOT a candidate): ordered Fe3W analog of the tier-1-clean Fe17W3 carrier (file 15265418-c679-4476-b784-caebd397df9b). a = 4.0804 A and interlayer 1.4568 A taken from Fe17W3; layers repeat Fe17W3's layer chemistry: mixed W/Fe checkerboard (W (0,0,0), Fe (0.5,0.5,0)) + pure Fe layer ((0,0.5,0.5),(0.5,0,0.5)). Validated 2026-09-06: P4/mmm #123 at symprec 0.01 and 0.1, 4 atoms, ordered, dmin 2.507 A, vol/atom 12.13 A3 (identical to Fe17W3), density 12.03 g/cm3. Pre-registered caveats: 25 vs 15 at% W; W-W 2.914 A along c where the carrier W has no W neighbor within 3.0 A.