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>

data/modernization_feature_flags.json

@@ -0,0 +1,21 @@
+{
+  "change": "full-modernization-architecture-blueprint",
+  "route_flags": {
+    "/wip-overview": {"content_cutover_enabled": true},
+    "/wip-detail": {"content_cutover_enabled": true},
+    "/hold-overview": {"content_cutover_enabled": true},
+    "/hold-detail": {"content_cutover_enabled": true},
+    "/hold-history": {"content_cutover_enabled": true},
+    "/resource": {"content_cutover_enabled": true},
+    "/resource-history": {"content_cutover_enabled": true},
+    "/qc-gate": {"content_cutover_enabled": true},
+    "/job-query": {"content_cutover_enabled": true},
+    "/tmtt-defect": {"content_cutover_enabled": true},
+    "/admin/pages": {"content_cutover_enabled": true},
+    "/admin/performance": {"content_cutover_enabled": true},
+    "/tables": {"content_cutover_enabled": false},
+    "/excel-query": {"content_cutover_enabled": false},
+    "/query-tool": {"content_cutover_enabled": false},
+    "/mid-section-defect": {"content_cutover_enabled": false}
+  }
+}