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

tests/fixtures/page-editor/feature-grid-v1.json

@@ -0,0 +1,35 @@
+{
+  "schemaVersion": 1,
+  "sections": [
+    {
+      "id": "fixture-feature-grid",
+      "type": "feature-grid",
+      "layoutVariant": "grid-3",
+      "colorScheme": "default",
+      "styles": {
+        "contentWidth": "contained"
+      },
+      "props": {
+        "heading": {
+          "type": "static",
+          "value": "Our Features"
+        },
+        "items": {
+          "type": "static",
+          "value": [
+            {
+              "title": "Fast setup",
+              "description": "Launch a storefront page with reusable sections.",
+              "icon": "Zap"
+            },
+            {
+              "title": "Consistent render",
+              "description": "Use one contract across editor, storefront, and SSR.",
+              "icon": "Layers"
+            }
+          ]
+        }
+      }
+    }
+  ]
+}