fix: define zero-column rank detection contract

[shawcharles]

Closes #577.

Summary

This PR defines _detect_rank_deficiency() behaviour for design matrices with zero columns.

Previously, _detect_rank_deficiency(np.empty((n, 0))) reached pivoted QR, received an empty QR diagonal, and raised an IndexError when indexing r_diag[0].

The new behaviour treats an (n, 0) design as rank 0, with no dropped columns and an empty pivot array. That gives the helper an explicit empty-design contract without changing rank-deficiency handling for any matrix with columns.

Changes

• Add an early return for k == 0 in _detect_rank_deficiency().
• Add a direct regression test for the zero-column contract.
• Keep existing rank-zero non-empty-column behaviour unchanged.

Validation

• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_linalg.py::TestNumericalStability::test_rank_detection_zero_column_matrix_returns_empty_contract -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_linalg.py::TestNumericalStability -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_linalg.py -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python .venv-py39/bin/python -m pytest tests/test_linalg.py::TestNumericalStability::test_rank_detection_zero_column_matrix_returns_empty_contract -q
• git diff --check

Security / privacy

Confirmed: no secrets or PII in this PR.

[shawcharles]

Ready for review from my side.

I rebased the branch onto current upstream/main and reran the focused checks:

• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_linalg.py::TestNumericalStability::test_rank_detection_zero_column_matrix_returns_empty_contract -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_linalg.py -q
• PYTHONPATH=. DIFF_DIFF_BACKEND=python .venv-py39/bin/python -m pytest tests/test_linalg.py::TestNumericalStability::test_rank_detection_zero_column_matrix_returns_empty_contract -q
• python -m ruff check diff_diff/linalg.py
• git diff --check upstream/main...HEAD

Scope note: this only defines the helper contract for an (n, 0) design matrix. It is not intended to change any estimator or statistical-method behaviour.

The current CI Gate failure is only the repository label gate:

The ready-for-ci label is required to run CI tests.

I do not have permission to apply that label, so this is ready whenever you want to label it.