feat: Page Editor v1.0 - canonical schema, SSR parity, and migration

Major improvements to WooNooW Page Editor system:

Schema & Architecture:
- Canonical section schema with unified sectionSchema.ts
- Normalized feature-grid to use items (not features)
- Standardized default values across all section types
- Schema versioning with automatic migration on read

Backend (PHP):
- Enhanced PlaceholderRenderer with typed output contracts
- Added fallback behavior for empty/invalid dynamic sources
- Added caching support for post data resolution
- New SchemaMigration class for backward compatibility
- New Features class for feature flags
- Enhanced PageSSR with full style support
- Removed controller-level special-casing for related_posts

Frontend (Admin SPA):
- Updated CanvasRenderer with schema-aware transformation
- Enhanced InspectorPanel with canonical schema metadata
- Added new section renderers

Frontend (Customer SPA):
- New section components: BentoCategoryGrid, MarqueeBanner, ProductCarousel, ShoppableImage
- Updated FeatureGridSection for items prop contract

Testing:
- Add PHP tests: SchemaMigrationTest, PlaceholderRendererTest, PageSSRTest
- Add TypeScript tests: schema-integration, feature-grid-regression
- Add parity tests for React vs SSR content matching
- Add CI script: check-schema-drift.mjs
- Add VERIFICATION_CHECKLIST.md

Documentation:
- RELEASE_NOTES-v1.0.md with full release notes
- docs/PAGE_EDITOR_SECTION_SCHEMA_V1.md
- docs/PAGE_EDITOR_SSR_COVERAGE_AUDIT.md

customer-spa/src/pages/DynamicPage/sections/FeatureGridSection.tsx

@@ -41,7 +41,9 @@ export function FeatureGridSection({
     };
     const customPadding = styles?.paddingTop || styles?.paddingBottom;
     const heightClasses = customPadding ? '' : (heightMap[styles?.heightPreset || 'default'] || 'py-12 md:py-20');
-    const listItems = items.length > 0 ? items : features;
+    const safeItems = Array.isArray(items) ? items : [];
+    const safeFeatures = Array.isArray(features) ? features : [];
+    const listItems = safeItems.length > 0 ? safeItems : safeFeatures;
 
     const gridCols = {
         'grid-2': 'md:grid-cols-2',