Bridge: fetch stable URL from MokoOnyx updates.xml, fix double-exec

- Remove hardcoded RELEASE_URL, discover from updates.xml stable channel
- Add discoverStableUrl() to parse MokoOnyx updates.xml at runtime
- Extract httpGet() helper for reuse across download + XML fetch
- Remove bridge call from update() — postflight() handles it
- Always targets stable channel for production-safe installs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/script.php

@@ -113,15 +113,7 @@ class Tpl_MokocassiopeiaInstallerScript
             );
         }
 
-        // Bridge migration: MokoCassiopeia → MokoOnyx
-        $bridgeScript = __DIR__ . '/helper/bridge.php';
-        if (is_file($bridgeScript)) {
-            require_once $bridgeScript;
-            if (class_exists('MokoBridgeMigration')) {
-                MokoBridgeMigration::run();
-            }
-        }
-
+        // Bridge migration runs in postflight() — not here — to avoid double execution
         return true;
     }