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).

app/core/vst_engine.py

@@ -49,27 +49,29 @@ def render_midi_events_to_audio(midi_events: list, sr: int = 44100, bpm: float =
         out_r /= max_peak
     return np.vstack([out_l, out_r])
 
-def check_pedalboard_safe():
-    import subprocess, sys
+def _module_available(name: str) -> bool:
+    """Kiem tra module co san khong — KHONG spawn subprocess.
+
+    Truoc day dung subprocess.run([sys.executable, '-c', 'import X']) —
+    khi app dong goi (PyInstaller frozen), sys.executable = daw_engine.exe
+    -> subprocess chay CA ENGINE (bootloader bo qua '-c', chay desktop_engine)
+    -> moi lan check lai sinh ra engine moi -> de quy spawn vo han
+    (Task Manager day daw_engine, port 8000-8005 leo thang, load rat cham).
+    find_spec() nhanh (micro-giay) va hoat dong ca source lan frozen.
+    """
+    import importlib.util
     try:
-        res = subprocess.run(
-            [sys.executable, "-c", "import pedalboard"],
-            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, timeout=2.0
-        )
-        return res.returncode == 0
-    except Exception:
+        return importlib.util.find_spec(name) is not None
+    except (ImportError, AttributeError, ValueError):
         return False
 
+
+def check_pedalboard_safe():
+    return _module_available("pedalboard")
+
+
 def check_pyfluidsynth_safe():
-    import subprocess, sys
-    try:
-        res = subprocess.run(
-            [sys.executable, "-c", "import fluidsynth"],
-            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, timeout=2.0
-        )
-        return res.returncode == 0
-    except Exception:
-        return False
+    return _module_available("fluidsynth")
 
 HAS_PEDALBOARD = check_pedalboard_safe()
 HAS_PYFLUIDSYNTH = check_pyfluidsynth_safe()