feat(js): implement intersection observer lazy loading for adsense

- Add per-slot lazy loading with Intersection Observer API
- Implement fill detection via MutationObserver and data-ad-status
- Add configurable rootMargin and fillTimeout from database
- Generate dynamic CSS based on lazy_loading_enabled setting
- Add legacy mode fallback for browsers without IO support
- Include backup of previous implementation (adsense-loader.legacy.js)
- Add OpenSpec documentation with test plan (72 tests verified)

Schema changes:
- Add lazy_loading_enabled (boolean, default: true)
- Add lazy_rootmargin (select: 0-500px, default: 200)
- Add lazy_fill_timeout (select: 3000-10000ms, default: 5000)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Admin/AdsensePlacement/Infrastructure/FieldMapping/AdsensePlacementFieldMapper.php

@@ -56,6 +56,11 @@ final class AdsensePlacementFieldMapper implements FieldMapperInterface
             'adsense-placementRailFormat' => ['group' => 'behavior', 'attribute' => 'rail_format'],
             'adsense-placementRailTopOffset' => ['group' => 'behavior', 'attribute' => 'rail_top_offset'],
 
+            // BEHAVIOR (Lazy Loading)
+            'adsense-placementLazyLoadingEnabled' => ['group' => 'behavior', 'attribute' => 'lazy_loading_enabled'],
+            'adsense-placementLazyRootmargin' => ['group' => 'behavior', 'attribute' => 'lazy_rootmargin'],
+            'adsense-placementLazyFillTimeout' => ['group' => 'behavior', 'attribute' => 'lazy_fill_timeout'],
+
             // LAYOUT (Archive/Global locations + formats)
             'adsense-placementArchiveTopEnabled' => ['group' => 'layout', 'attribute' => 'archive_top_enabled'],
             'adsense-placementArchiveBetweenEnabled' => ['group' => 'layout', 'attribute' => 'archive_between_enabled'],