Validate the subspace constraint for spherical tangent vectors - #267

For a sphere restricted by intersection or complement, a tangent vector must belong to the allowed subspace as well as being orthogonal to the base point. The tangent checker currently only tests orthogonality, so it accepts vectors normal to the allowed subspace.

Check that the vector is unchanged by the existing subspace projection. Preserve the existing orthogonality failure and expose the violated subspace constraint through the assertion message.

Validation on current master with Python 3.12 / PyTorch 2.8.0 CPU and full native package imports:

• The same 7 focused cases give 4 failures and 3 passing controls on master, then all 7 pass with the patch.
• All original test_manifold_basic.py cases plus the focused file: 1,897 passed, 310 existing parameterized skips.
• Cases cover intersection/complement constraints, scalar and mixed batches, projected vectors, zero vectors, and the unconstrained sphere.
• Production Ruff, focused-test Ruff and git diff --check pass.

The branch preserves both new master commits. The complete repository suite, CUDA and the Sphinx documentation gate were not run for this change.