Remove Carla native VSTi bridge entirely

Drop Carla integration across frontend and backend:
- plugins.py: remove /carla-midi, /carla-status, /carla-stop, /open-in-carla,
  /carla-play-notes endpoints and Carla process management
- runtime.py: remove find_carla / _carla_* helpers, detect and capabilities
- app.jsx: remove Carla bridge UI, SonicCarlaMidi routing, shouldRouteCarla
  guards, VST row Carla button and preset fallback
- system.py: remove /carla-path
- Keep audio path unchanged (tracks -> masterBus -> output -> analyser),
  directOut only for offline render

Build frontend, tests pass (124 passed).

native_bridge/tests/g2_bridge_smoke.py

@@ -102,7 +102,7 @@ try:
         logf.flush()
         print("bridge out:", open(logpath, encoding="utf-8").read()[-1500:])
 
-    # VST3 (native hosting, no Carla): mda-vst3 bundle from SDK build
+    # VST3 (native hosting): mda-vst3 bundle from SDK build
     VST3 = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "build", "VST3", "Release", "mda-vst3.vst3"))
     load_instrument(0, VST3, ch=2)
     # wait until bridge reports the load (poll log), up to 10s