v0.12.0 — certified demagnetization-tensor reference tables

Every finite-difference micromagnetic simulator (OOMMF, MuMax3, magnum.np, Fidimag, MagTense) computes the demagnetizing field from the same Newell tensor, whose closed-form evaluation is known to lose all significant digits to catastrophic cancellation at large cell separations. This release pins the entries rigorously: two-sided enclosures of dyadic rationals from outward-rounded interval arithmetic, with the transcendental pieces enclosed by truncated series. Against them, the naive double-precision route is measured losing about six correct decimal digits per decade of separation — zero correct digits near 300 cells, wrong sign and magnitude beyond — and the crossover with OOMMF's asymptotic expansion is located rigorously. Certificate JSON plus a standard-library checker that re-derives every enclosure and rejects tampering. Review log: demag-paper/FIXLOG.md. Paper: demag-paper/note.pdf