Threefold walls

The four-coordinate compressed model of a polarised threefold and its Bayer--Macrì--Toda charge, as a child of Walls/Alignment.lean. This is the threefold counterpart of Walls/Numerical/, which does the same for surfaces in three coordinates.

Declarations use the strong-child namespace CategoryTheory.Triangulated.WeakStabilityCondition.StabilityCondition.Wall, inside a Threefold namespace so that its NumClass, reZ and imZ do not collide with the surface ones.

The BMT inequality is not asserted anywhere in this subtree; it is false in general, and AlgebraicGeometry/Numerical/Stability/BMT.lean carries it as supplied data with that warning.