feat(coverage): pattern-library coverage measurement (#2692, milestone 288 step 7)

Server-side shape enumeration per bound repo — one archive download via the
forge adapter, definitions extracted with a Python mirror of the write-path
hook's awk rules (shared test vectors pin the two together) — compared
against recorded snippet locations by path+symbol. Summary is cached in the
settings KV with a freshness stamp; recomputed on webhook push (spawned off
the delivery path) or explicit refresh, never in a request path.

Surfaces: GET/POST /api/projects/<id>/coverage[/refresh], a project-page
card (estimate-labeled, largest-gaps chips), and a one-line evidence-carrying
entry in enter_project read from cache only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_mcp_tool_projects.py

@@ -29,6 +29,17 @@ def _no_systems():
         yield
 
 
+@pytest.fixture(autouse=True)
+def _no_coverage():
+    """enter_project also reads the pattern-coverage cache (#2692) — same
+    deal: no database here, stub the common case (nothing computed). The
+    populated line is asserted in tests/test_pattern_coverage.py.
+    """
+    with patch("scribe.mcp.tools.projects.coverage_svc.cached_coverage",
+               AsyncMock(return_value=None)):
+        yield
+
+
 def _fake_project(design_system_id=None, **overrides) -> MagicMock:
     p = MagicMock()
     base = {"id": 1, "title": "P", "description": "", "goal": "",