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>

Inc/enqueue-scripts.php

@@ -489,6 +489,18 @@ function roi_enqueue_adsense_loader() {
             'strategy'  => 'defer',
         )
     );
+
+    // Pasar configuración de lazy loading a JavaScript
+    $lazy_enabled = roi_get_component_setting('adsense-placement', 'behavior', 'lazy_loading_enabled', true);
+    $lazy_rootmargin = roi_get_component_setting('adsense-placement', 'behavior', 'lazy_rootmargin', '200');
+    $lazy_fill_timeout = roi_get_component_setting('adsense-placement', 'behavior', 'lazy_fill_timeout', '5000');
+
+    wp_localize_script('roi-adsense-loader', 'roiAdsenseConfig', array(
+        'lazyEnabled' => (bool) $lazy_enabled,
+        'rootMargin' => (int) $lazy_rootmargin . 'px 0px',
+        'fillTimeout' => (int) $lazy_fill_timeout,
+        'debug' => defined('WP_DEBUG') && WP_DEBUG,
+    ));
 }
 
 add_action('wp_enqueue_scripts', 'roi_enqueue_adsense_loader', 10);