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).

app/static/js/services/unifiedMidiRouter.js

@@ -1,7 +1,7 @@
 // app/static/js/services/unifiedMidiRouter.js
 // Single MIDI entry point for Web MIDI keyboard + timeline playback.
 // bridge connected -> NativeBridgeService.dispatchMidiEvent (Rust SHM -> C++ bridge)
-// bridge down      -> onFallback callback (app.jsx wires SonicSF/Carla path).
+// bridge down      -> onFallback callback (app.jsx wires SonicSF/native path).
 (function () {
   var MELODIC_CHANNELS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14]; // skip 9 (percussion); 15 reserved for Media Explorer preview