feat(php): implement cache-first architecture hook

Add CacheFirstHooksRegistrar that fires roi_theme_before_page_serve
hook on template_redirect priority 0 for singular pages.

- Only fires for anonymous users (cache doesn't apply to logged in)
- Only fires for singular pages (posts, pages, CPTs)
- Provides post_id to external plugins
- Does NOT define DONOTCACHEPAGE (allows page caching)

Plan 1000.01 implementation.

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

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

functions.php

@@ -174,6 +174,12 @@ try {
     );
     $youtubeFacadeHooksRegistrar->register();
 
+    // === CACHE-FIRST ARCHITECTURE (Plan 1000.01) ===
+    // Hook para plugins externos que necesitan evaluar acceso antes de servir página
+    // @see openspec/specs/cache-first-architecture/spec.md
+    $cacheFirstHooksRegistrar = new \ROITheme\Shared\Infrastructure\Hooks\CacheFirstHooksRegistrar();
+    $cacheFirstHooksRegistrar->register();
+
     // Log en modo debug
     if (defined('WP_DEBUG') && WP_DEBUG) {
         error_log('ROI Theme: Admin Panel initialized successfully');