perf(win): skip DWM blur/acrylic when the window starts opaque

The window always asked DWM for a full-window blur region and, on
Windows 11, an acrylic backdrop. When the user runs with an opaque
window (low-spec mode or full opacity) that desktop blur is never seen
but DWM still composites it every frame.

- An early read of the saved settings decides whether the window starts
  opaque before the backdrop is configured.
- When it does, the full-window blur region is left as SDL's zero-area
  default and the backdrop type is set to none instead of acrylic.

Windows-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -986,11 +986,13 @@ int main(int argc, char* argv[])
     int savedWinW = 1200, savedWinH = 720;
     float earlyFontScale = 1.0f;      // consumed below, before the first (and only) font-atlas build
     bool  earlyEmojiColor = false;
+    bool  earlyOpaque = false;        // opaque start: skip DWM blur/acrylic backdrop (Windows)
     {
         dragonx::config::Settings earlySettings;
         const bool earlyLoaded = earlySettings.load();
         earlyFontScale  = earlySettings.getFontScale();        // defaults when load failed, same as App::init sees
         earlyEmojiColor = earlySettings.getChatEmojiColor();
+        earlyOpaque = earlySettings.getLowSpecMode() || earlySettings.getWindowOpacity() >= 0.99f;
         if (earlyLoaded) {
             int sw = earlySettings.getWindowWidth();
             int sh = earlySettings.getWindowHeight();
@@ -1163,6 +1165,8 @@ int main(int argc, char* argv[])
         if (hwnd_blur) {
             // Override SDL's zero-area blur region with a full-window blur.
             // This gives DWM the signal to blur everything behind our window.
+            // Skipped when the window starts opaque (SDL's zero-area region stays).
+            if (!earlyOpaque) {
                 HRGN blurRgn = CreateRectRgn(0, 0, -1, -1);  // full window
                 DWM_BLURBEHIND bb = {};
                 bb.dwFlags = DWM_BB_ENABLE | DWM_BB_BLURREGION;
@@ -1171,6 +1175,7 @@ int main(int argc, char* argv[])
                 HRESULT hr_blur = DwmEnableBlurBehindWindow(hwnd_blur, &bb);
                 DeleteObject(blurRgn);
                 DEBUG_LOGF("DwmEnableBlurBehindWindow (full region): 0x%08lx\n", hr_blur);
+            }
 
             // On Windows 11 22H2+, also enable Acrylic for nicer modern blur
             auto ver = dragonx::platform::getWindowsVersion();
@@ -1188,7 +1193,7 @@ int main(int argc, char* argv[])
 
                 // Acrylic = blurred, semi-transparent desktop view
                 // (NOT Mica which is opaque)
-                DWORD backdrop = 3;  // DWMSBT_TRANSIENTWINDOW = Acrylic
+                DWORD backdrop = earlyOpaque ? 1 /*DWMSBT_NONE*/ : 3 /*DWMSBT_TRANSIENTWINDOW = Acrylic*/;
                 HRESULT hr_acr = DwmSetWindowAttribute(hwnd_blur, 38,
                     &backdrop, sizeof(backdrop));
                 DEBUG_LOGF("DWM Acrylic (DWMSBT_TRANSIENTWINDOW): 0x%08lx\n", hr_acr);