Volume constructors accept non-invertible affine matrices #450

Describe the bug

Volume and VolumeGeometry can accept a rank-deficient affine matrix when a dependent column is small enough to pass the approximate scaled-orthogonality check. The object is then invalid for later spatial operations that require inversion.

To reproduce

Start from an identity affine and make one spatial column a small multiple of another, for example:

affine = np.eye(4)
affine[:3, 2] = affine[:3, 0] * 1e-6
Volume(np.zeros((2, 2, 2)), affine, coordinate_system="PATIENT")

Current behavior: construction succeeds, but downstream operations may fail at np.linalg.inv().

Expected behavior

The public volume constructors should reject non-invertible affine matrices immediately with a clear ValueError, while continuing to accept genuinely invertible scaled-orthogonal affines.

Scope

Validate invertibility at the shared constructor boundary, update the affine contract documentation, and cover both Volume and VolumeGeometry.

Process note

I am filing this issue before the replacement pull request in accordance with the developer guide's issue-first recommendation. PR #435 was opened before that preflight and will be superseded.