bridge: fix Qin VST2 silent playback via per-child SHM sandbox

- out-of-process VST2 sandbox: each Qin track spawns its own plugin_host.exe child
  with unique SHM (SonicForge_PluginHost2_<bridgePid>_<ch>_<seq>), watchdog <=3 respawns
- preset round-trip bridge<->child over SHM (presetIn/presetOut 64KB), stub rejection MIN_PRESET=100
- seed qin_default.b64 default preset into every fresh child; saved patch wins on restore
- forward MIDI on channel 0 (Qin_RV only listens on ch0); one instrument per child
- controls 4/5/6 = OPEN_GUI/CLOSE_GUI/QUIT
- gitignore runtime carla_projects

src-tauri/tauri.conf.json

@@ -27,7 +27,7 @@
       "binaries/libfluidsynth-3.dll": "libfluidsynth-3.dll",
       "binaries/jpeg62.dll": "jpeg62.dll"
     },
-    "externalBin": ["binaries/daw_vst_bridge", "binaries/fx_vst_bridge"],
+    "externalBin": ["binaries/daw_vst_bridge", "binaries/fx_vst_bridge", "binaries/plugin_host"],
     "windows": {
       "nsis": {
         "installerHooks": "hooks.nsh"