3+1 spacetime signature

The fibered shape already separates the base τ^1 from the T^2-fiber. After global Cartesian gluing, the T^2-derived bulk supplies the spatial readout, while the τ^1-base supplies the temporal direction that Book V will develop as proper time. The resulting 3+1 signature is therefore not chosen as a background Lorentzian metric. It is the structural signature of the carrier: three spatial directions from the Hartogs bulk, one temporal base direction from τ^1.