test(audit): add dedicated test suite for a11y sub-check (follow-up #256)

docs/design/0256-restore-a11y.md

@@ -54,10 +54,20 @@ count at exactly 12.
 
 ## Testing
 
-Verified end-to-end via CLI: `codelens audit tests --check a11y` → 2 real
-findings (missing_label high + semantic_html low) from `tests/fixtures/sample.html`.
-`--severity high` passthrough confirmed. `tests/test_command_registry.py`
-2 passed.
+`tests/test_a11y_command.py` (6 tests): wrapper delegation,
+severity/category passthrough, audit umbrella dispatch of `a11y`, category
+reaching the engine through the synthetic namespace, and a regression guard
+that `css` (#251) and `a11y` coexist independently (neither shadowing the
+other in `_CHECKS`).
+
+Verified end-to-end on the real Coretax `smart-tax-assistance` workspace
+(not just a fixture): `audit . --check a11y` → **165 findings** across
+semantic_html (94), link_text (35), missing_label (24), keyboard_nav (5),
+missing_alt (4), color_contrast (3). `--category missing_alt` correctly
+narrows to 4, confirming passthrough reaches the engine.
+
+The engine was confirmed working *before* wiring (per the issue's
+constraint), so this PR is pure entry-point restoration.
 
 ## Alternatives Considered
 

tests/test_a11y_command.py

@@ -0,0 +1,86 @@
+# @WHO:   tests/test_a11y_command.py
+# @WHAT:  Tests for restored a11y as audit --check a11y (issue #256)
+# @PART:  tests
+"""Tests for the a11y sub-check of the audit umbrella (issue #256).
+
+a11y_engine was orphaned in the #195 consolidation (its command entry
+point was deleted, the engine kept) — the same situation as css-deep
+(#251) and export-snapshot (#218). This restores access as
+`audit --check a11y` — a sub-check, NOT a new top-level command.
+"""
+
+import argparse
+import os
+import sys
+from unittest import mock
+
+import pytest
+
+SCRIPT_DIR = os.path.join(
+    os.path.dirname(os.path.dirname(os.path.abspath(__file__))), "scripts"
+)
+if SCRIPT_DIR not in sys.path:
+    sys.path.insert(0, SCRIPT_DIR)
+
+from commands import a11y  # noqa: E402
+from commands import audit  # noqa: E402
+
+
+class TestA11yCommand:
+    def test_execute_delegates_to_engine(self):
+        args = argparse.Namespace(workspace=".", severity=None, category=None)
+        with mock.patch(
+            "commands.a11y.audit_accessibility",
+            return_value={"status": "ok", "stats": {"total_issues": 3}},
+        ) as mock_engine:
+            result = a11y.execute(args, ".")
+        mock_engine.assert_called_once_with(".", category=None, severity=None)
+        assert result["status"] == "ok"
+
+    def test_severity_and_category_passthrough(self):
+        args = argparse.Namespace(workspace=".", severity="high", category="missing_alt")
+        with mock.patch(
+            "commands.a11y.audit_accessibility",
+            return_value={"status": "ok"},
+        ) as mock_engine:
+            a11y.execute(args, ".")
+        mock_engine.assert_called_once_with(".", category="missing_alt", severity="high")
+
+
+class TestAuditDispatchesA11y:
+    def test_a11y_is_registered_check(self):
+        assert "a11y" in audit.ALL_CHECKS
+
+    def test_audit_check_a11y_routes_to_engine(self):
+        base = argparse.Namespace(
+            workspace=".", check="a11y", severity=None, category=None,
+        )
+        with mock.patch(
+            "commands.a11y.audit_accessibility",
+            return_value={"status": "ok", "stats": {"total_issues": 9}},
+        ):
+            result = audit.execute(base, ".")
+        assert result["s"] == "ok"
+        assert result["st"]["checks_run"] == 1
+        assert result["r"][0]["_check"] == "a11y"
+        assert result["r"][0]["stats"]["total_issues"] == 9
+
+    def test_audit_check_a11y_category_reaches_engine(self):
+        base = argparse.Namespace(
+            workspace=".", check="a11y", severity=None, category="missing_label",
+        )
+        with mock.patch(
+            "commands.a11y.audit_accessibility",
+            return_value={"status": "ok"},
+        ) as mock_engine:
+            audit.execute(base, ".")
+        _, kwargs = mock_engine.call_args
+        assert kwargs.get("category") == "missing_label"
+
+    def test_css_and_a11y_both_registered_independently(self):
+        """Regression guard: a11y was added alongside the existing css
+        sub-check (#251) — both must coexist, neither shadowing the other."""
+        assert "css" in audit.ALL_CHECKS
+        assert "a11y" in audit.ALL_CHECKS
+        assert audit._CHECKS["a11y"]["module"] == "commands.a11y"
+        assert audit._CHECKS["css"]["module"] == "commands.css_deep"