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/test_feature_flags.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+"""Tests for shared feature-flag resolution helpers."""
+
+from __future__ import annotations
+
+from mes_dashboard.core.feature_flags import parse_bool, resolve_bool_flag
+
+
+def test_parse_bool_supports_true_and_false_tokens():
+    assert parse_bool("true", default=False) is True
+    assert parse_bool(" yes ", default=False) is True
+    assert parse_bool("0", default=True) is False
+    assert parse_bool("off", default=True) is False
+
+
+def test_resolve_bool_flag_prefers_environment_over_config():
+    env = {"FEATURE_X": "false"}
+    config = {"FEATURE_X": True}
+    assert resolve_bool_flag("FEATURE_X", config=config, default=True, environ=env) is False
+
+
+def test_resolve_bool_flag_uses_config_then_default_when_env_missing():
+    config = {"FEATURE_X": "true"}
+    assert resolve_bool_flag("FEATURE_X", config=config, default=False, environ={}) is True
+    assert resolve_bool_flag("MISSING", config=config, default=False, environ={}) is False