Spec test gate misses module-level skips and missing test_spec modules #203

Release blocker found by the external review of #183 (ChatGPT batch 25, C2).

tests/spec_gate.py:18-40 only marks test items that have already been collected (pytest_collection_modifyitems), and only turns their skipped pytest_runtest_makereport results into failures. Two cases get through:

• Module-level skips: pytest.skip(..., allow_module_level=True) or pytest.importorskip(...) in a test_spec_*.py module runs during collection, before any item exists. The whole module is reported as skipped and the run still succeeds.
• Deleted modules: a deleted test_spec_*.py module is not detected.

The self-test (tests/test_spec_gate.py:12-57) covers only skips that happen after a test item exists.

Fix: add a collection-report hook (pytest_collectreport) that fails on a skipped test_spec_* module. Add a session-level assertion that the expected Spec modules and items were actually collected. Add regression tests for a module-level skip, a module-level importorskip, and a missing module.