fix: Update activator to use new HVAC_Route_Manager class

- Replace deprecated register_legacy_rewrite_rules() call with HVAC_Route_Manager
- Ensures plugin activation works with new architecture

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

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

includes/class-hvac-activator.php

@@ -40,10 +40,10 @@ class HVAC_Activator {
         // Schedule cron jobs
         self::schedule_cron_jobs();
         
-        // Register legacy redirects before flushing
-        if (class_exists('HVAC_Plugin')) {
-            $plugin = HVAC_Plugin::instance();
-            $plugin->register_legacy_rewrite_rules();
+        // Register routes before flushing
+        if (class_exists('HVAC_Route_Manager')) {
+            $route_manager = HVAC_Route_Manager::instance();
+            $route_manager->register_rewrite_rules();
         }
         
         // Flush rewrite rules