FSR2 temporal upscaling fixes: unjittered reprojection, sharpen Y-flip, MSAA guard, descriptor double-buffering

- Motion vectors: single unjittered reprojection matrix (80 bytes) instead of
  two jittered matrices (160 bytes), eliminating numerical instability from
  jitter amplification through large world coordinates
- Sharpen pass: fix Y-flip for correct UV sampling, double-buffer descriptor
  sets to avoid race with in-flight command buffers
- MSAA: auto-disable when FSR2 enabled, grey out AA setting in UI
- Accumulation: variance-based neighborhood clamping in YCoCg space,
  correct history layout transitions
- Frame index: wrap at 256 for stable Halton sequence

src/ui/game_screen.cpp

@@ -6281,7 +6281,13 @@ void GameScreen::renderSettingsWindow() {
                 }
                 {
                     const char* aaLabels[] = { "Off", "2x MSAA", "4x MSAA", "8x MSAA" };
-                    if (ImGui::Combo("Anti-Aliasing", &pendingAntiAliasing, aaLabels, 4)) {
+                    bool fsr2Active = renderer && renderer->isFSR2Enabled();
+                    if (fsr2Active) {
+                        ImGui::BeginDisabled();
+                        int disabled = 0;
+                        ImGui::Combo("Anti-Aliasing (FSR2)", &disabled, "Off (FSR2 active)\0", 1);
+                        ImGui::EndDisabled();
+                    } else if (ImGui::Combo("Anti-Aliasing", &pendingAntiAliasing, aaLabels, 4)) {
                         static const VkSampleCountFlagBits aaSamples[] = {
                             VK_SAMPLE_COUNT_1_BIT, VK_SAMPLE_COUNT_2_BIT,
                             VK_SAMPLE_COUNT_4_BIT, VK_SAMPLE_COUNT_8_BIT