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

docs/PAGE_EDITOR_SSR_COVERAGE_AUDIT.md

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+# Page Editor SSR Coverage Audit
+
+## Scope
+This audit compares section types available in the admin canvas and customer DynamicPage renderer against explicit PHP SSR renderers in `includes/Frontend/PageSSR.php`.
+
+## Coverage Matrix
+| Section type | Admin canvas | Customer SPA | PHP SSR | Status |
+|---|---:|---:|---:|---|
+| `hero` | yes | yes | `render_hero` | covered |
+| `content` | yes | yes | `render_content` | covered |
+| `image-text` | yes | yes | `render_image_text` | covered |
+| `feature-grid` | yes | yes | `render_feature_grid` | covered, now reads `items` with `features` fallback |
+| `cta-banner` | yes | yes | `render_cta_banner` | covered |
+| `contact-form` | yes | yes | `render_contact_form` | covered for SEO structure only |
+| `bento-category-grid` | yes | yes | `render_bento_category_grid` | covered |
+| `product-carousel` | yes | yes | `render_product_carousel` | covered for configured/static product data |
+| `shoppable-image` | yes | yes | `render_shoppable_image` | covered |
+| `marquee-banner` | yes | yes | `render_marquee_banner` | covered |
+
+## Immediate Finding
+The SSR path now has explicit renderers for all section types exposed by the editor canvas. The next gap is deeper content parity for sections that rely on runtime API data, especially product carousel data loading.
+
+## Next Implementation Targets
+1. Add product data resolution for `product-carousel` when SSR runs without pre-resolved products.
+2. Add parity fixtures covering one scalar-heavy section and one repeater-heavy section.
+3. Add snapshot tests comparing React content output vs SSR content output.