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/page-editor-fixtures.test.mjs

@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+
+const fixtureDir = resolve('tests/fixtures/page-editor');
+
+function readFixture(name) {
+  return JSON.parse(readFileSync(resolve(fixtureDir, name), 'utf8'));
+}
+
+function assertSectionProp(prop, expectedType = 'static') {
+  assert.equal(typeof prop, 'object');
+  assert.equal(prop.type, expectedType);
+  assert.ok('value' in prop || 'source' in prop);
+}
+
+describe('page editor fixtures', () => {
+  it('uses canonical feature-grid.items for v1 fixtures', () => {
+    const fixture = readFixture('feature-grid-v1.json');
+    const section = fixture.sections[0];
+
+    assert.equal(fixture.schemaVersion, 1);
+    assert.equal(section.type, 'feature-grid');
+    assertSectionProp(section.props.heading);
+    assertSectionProp(section.props.items);
+    assert.ok(Array.isArray(section.props.items.value));
+    assert.equal(section.props.features, undefined);
+  });
+
+  it('keeps a legacy feature-grid.features fixture for migration coverage', () => {
+    const fixture = readFixture('feature-grid-legacy-features.json');
+    const section = fixture.sections[0];
+
+    assert.equal(fixture.schemaVersion, 0);
+    assert.equal(section.type, 'feature-grid');
+    assertSectionProp(section.props.heading);
+    assertSectionProp(section.props.features);
+    assert.ok(Array.isArray(section.props.features.value));
+  });
+});