feat(modernization): full architecture blueprint with hardening follow-up

Implement phased modernization infrastructure for transitioning from
multi-page legacy routing to SPA portal-shell architecture, plus
post-delivery hardening fixes for policy loading, fallback consistency,
and governance drift detection.

Key changes:
- Add route contract enrichment with scope/visibility/compatibility policies
- Canonical 302 redirects from legacy direct-entry to /portal-shell/ routes
- Asset readiness enforcement and runtime fallback retirement for in-scope routes
- Shared feature-flag helpers (env > config > default) replacing duplicated _to_bool
- Defensive copy for lru_cached policy payloads preventing mutation corruption
- Unified retired-fallback response helper across app and blueprint routes
- Frontend/backend route-contract cross-validation in governance gates
- Shell CSS token fallback values for routes rendered outside shell scope
- Local-safe .env.example defaults with production recommendation comments
- Legacy contract fallback warning logging and single-hop redirect optimization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -22,15 +22,24 @@ os.environ.setdefault('WATCHDOG_STATE_FILE', os.path.join(_TMP_DIR, 'mes_dashboa
 
 import mes_dashboard.core.database as db
 from mes_dashboard.app import create_app
+from mes_dashboard.core.modernization_policy import clear_modernization_policy_cache
 
 
-@pytest.fixture
-def app():
+@pytest.fixture
+def app():
     """Create application for testing."""
     db._ENGINE = None
     app = create_app('testing')
-    app.config['TESTING'] = True
-    return app
+    app.config['TESTING'] = True
+    return app
+
+
+@pytest.fixture(autouse=True)
+def _reset_modernization_policy_cache():
+    """Keep policy-cache state isolated across tests."""
+    clear_modernization_policy_cache()
+    yield
+    clear_modernization_policy_cache()
 
 
 @pytest.fixture