v0.12.1 — erratum: gold-value anchor corrected from containment to agreement

Erratum to v0.12.0. The sixteen Maple gold values in OOMMF's demagcoef.cc agree with the certified enclosure midpoints to at least 49.6 digits; as first released the paper asserted they lie inside the enclosures, which is false — the enclosures are ~77 digits tight, so fifteen of the sixteen lie outside, the exact zero being the exception. The build log had already recorded the correct test (FIXLOG decision S4); the correction reached the anchor code but not the prose. No certified enclosure, certificate, or checker changed. Dated erratum footnote in the note; root cause and the resulting propagation gate recorded in the program protocol.