V.T25 — Minkowski as chart shadow --- cf. III.D76

For each base point p in tau^1, the local tau^3 chart R_p produces a Minkowski neighborhood: the readout algebra O_p is isomorphic to a neighborhood of the origin in R^{1,3} with the standard Minkowski metric eta_{mu nu} = diag(-1,+1,+1,+1).