fix: dau_engine de quy spawn vo han + load rat cham (ban 1.1.2)

Trieu chung tu user: Task Manager day daw_engine process, port 8000->8005
leo thang, engine.log co engine moi moi ~2.5-3.3s trong nhieu phut.

Goc re: app/core/vst_engine.py module-level:
  HAS_PEDALBOARD = check_pedalboard_safe()
  -> subprocess.run([sys.executable, '-c', 'import pedalboard'], timeout=2.0)
Khi frozen, sys.executable = daw_engine.exe. PyInstaller bootloader bo qua
'-c' va chay script nhung (desktop_engine.py = CA ENGINE) -> moi lan check
thu vien lai khoi dong mot engine moi (timeout 2s moi check, block startup
~4s) -> engine con import app.main -> lai chay check -> spawn tiep
=> de quy vo han, hang chuc engine, may nghen, load 6-7.5s.

Fix:
- vst_engine: check_pedalboard_safe/check_pyfluidsynth_safe dung
  importlib.util.find_spec (micro-giay, khong spawn, hoat dong frozen).
- desktop_engine.main(): guard phong thu - neu bi goi voi argv[1]=='-c'
  (dang bi dung nhu python interpreter) thi thoat ngay khong chay server.

Verify tren Linux (engine frozen):
- STARTUP: 6-7.5s -> 1.24-1.33s (3 lan lien tiep)
- sau 15s: dung 1 engine, 0 process '-c import' (truoc: hang chuc)
- 86 tests pass; test suite 34s -> 14s (may het bi spam).

desktop_engine.py

@@ -52,6 +52,13 @@ def _parent_alive(pid):
 
 
 def main():
+    # PHONG THU (fix ban 1.1.2): neu exe bi goi nhu 'python -c ...' — vd code
+    # cu kiem tra thu vien bang subprocess.run([sys.executable, '-c', 'import X'])
+    # thi frozen sys.executable = daw_engine.exe -> se chay CA ENGINE o day.
+    # Thoat ngay, KHONG khoi dong server, tranh de quy spawn vo han.
+    if len(sys.argv) > 1 and sys.argv[1] == "-c":
+        return
+
     os.environ.setdefault("SF_DESKTOP", "1")
 
     # Log dir tao SOM de stderr co the tro vao file (xem duoi).