Fix GUI Qin không hiện lại sau hide + âm thanh lỗi sau khi đóng cửa sổ

- plugin_host_main.cpp: chặn WM_CLOSE (đóng GUI nhưng giữ window sống, hide
  thay vì destroy), WM_DESTROY reset hwnd; dùng g_hostHwnd toàn cục thay
  hwnd local để control OPEN_GUI/CLOSE_GUI tái sử dụng đúng window.
- main.cpp: bỏ qua dedupe hasAttachedView cho instrument child-own
  (SandboxVst2Host) — bridge guiOpen_ stale sau khi child tự đóng nên
  OPEN_GUI lặp lại phải forward control 4; child tự chặn double-open.
- Verify: WM_CLOSE -> window ẩn còn sống; OPEN_GUI lần 2 -> window hiện lại,
  âm ch1 vẫn kêu (peak 0.15), không câm.

native_bridge/src/plugin_host_main.cpp

@@ -38,10 +38,41 @@ static bool parent_alive(uint32_t pid) {
 #endif
 }
 
+// G4.7: the child owns the host window, so the user's X click lands HERE, not
+// on the bridge. DefWindowProc destroys the window on WM_CLOSE while the VST2
+// instrument still thinks its editor is attached (guiAttached_ stale true) -
+// a second OPEN_GUI then can never re-show the GUI (bridge dedupes on
+// hasAttachedView, and the child's openGUI guard skips too). Intercept:
+// WM_CLOSE -> effEditClose + hide (keep the window alive, reopen works);
+// WM_DESTROY (external destroy) -> forget the hwnd so the next OPEN_GUI
+// creates a fresh window instead of attaching to a dead one.
+static INativeInstrument* g_wndInst = nullptr;
+static HWND g_hostHwnd = nullptr;
+
+static LRESULT CALLBACK PluginHostWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
+    switch (msg) {
+        case WM_CLOSE:
+            if (g_wndInst) {
+                try { g_wndInst->closeGUI(); } catch (...) {}
+            }
+            if (g_hostHwnd) ShowWindow(g_hostHwnd, SW_HIDE);
+            return 0;  // swallow: do NOT let DefWindowProc destroy the window
+        case WM_DESTROY:
+            if (g_wndInst) {
+                try { g_wndInst->closeGUI(); } catch (...) {}
+            }
+            g_hostHwnd = nullptr;
+            break;
+        default:
+            break;
+    }
+    return DefWindowProc(hwnd, msg, wParam, lParam);
+}
+
 static HWND make_host_window() {
     WNDCLASSEX wc = {};
     wc.cbSize = sizeof(wc);
-    wc.lpfnWndProc = DefWindowProc;
+    wc.lpfnWndProc = PluginHostWndProc;
     wc.hInstance = GetModuleHandle(nullptr);
     wc.lpszClassName = "PluginHostClass";
     RegisterClassEx(&wc);
@@ -106,6 +137,7 @@ int main(int argc, char* argv[]) {
         }
     }
     INativeInstrument& inst = vst2 ? (INativeInstrument&)instV2 : (INativeInstrument&)instV3;
+    g_wndInst = &inst;  // WM_CLOSE (user X) needs the live instrument
 
     // G4.6 auto-play fix: a loaded chunk can embed a stuck/sustained note
     // (factory state, seed, or a patch captured mid-note). Release everything
@@ -145,12 +177,12 @@ int main(int argc, char* argv[]) {
     // VST3: GUI on the main thread at startup (editorhost pattern: attach then
     // pump below). VST2: deferred until control 4 (OPEN_GUI) so loading a Qin
     // does not pop a window for every track.
-    HWND hwnd = nullptr;
+    g_hostHwnd = nullptr;
     if (!vst2) {
-        hwnd = make_host_window();
-        printf("[plugin_host] ownHwnd=%p openGUI...\n", (void*)hwnd);
+        g_hostHwnd = make_host_window();
+        printf("[plugin_host] ownHwnd=%p openGUI...\n", (void*)g_hostHwnd);
         fflush(stdout);
-        bool gui = inst.openGUI(hwnd);
+        bool gui = inst.openGUI(g_hostHwnd);
         printf("[plugin_host] openGUI=%d\n", gui ? 1 : 0);
         fflush(stdout);
     }
@@ -208,16 +240,16 @@ int main(int argc, char* argv[]) {
             for (uint32_t i = 0; i < nctl; ++i) {
                 const auto& c = ipc->base.controlQueue[i];
                 if (c.type == 4) {
-                    if (!hwnd) hwnd = make_host_window();
-                    if (hwnd && !inst.hasAttachedView()) {
+                    if (!g_hostHwnd) g_hostHwnd = make_host_window();
+                    if (g_hostHwnd && !inst.hasAttachedView()) {
                         printf("[plugin_host] OPEN_GUI ch=%u\n", channel);
                         fflush(stdout);
-                        inst.openGUI(hwnd);
-                        ShowWindow(hwnd, SW_SHOW);
+                        inst.openGUI(g_hostHwnd);
+                        ShowWindow(g_hostHwnd, SW_SHOW);
                     }
                 } else if (c.type == 5) {
                     inst.closeGUI();
-                    if (hwnd) ShowWindow(hwnd, SW_HIDE);
+                    if (g_hostHwnd) ShowWindow(g_hostHwnd, SW_HIDE);
                 } else if (c.type == 6) {
                     quit = true;
                 }