perf(resize): dedupe the live-resize watcher, early-out DX11 resize

The live-resize event watcher redrew for every resize event, and each
resize tore down and recreated the DX11 render target even when the
pixel size had not actually changed. A single drag fires both a RESIZED
and a PIXEL_SIZE_CHANGED event reporting the same final size, so the
work ran twice.

- The watcher keys off the real pixel size and returns early when it has
  not moved since the last pass, collapsing the duplicate event pair into
  one redraw. It also bails out when called from inside an open ImGui
  frame, which the caption buttons can trigger.
- The DX11 resize now checks the swap chain's current dimensions up
  front and does nothing when the size already matches and the target is
  live, skipping the destroy/recreate churn.

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

src/main.cpp

@@ -1527,6 +1527,20 @@ int main(int argc, char* argv[])
         if (event->window.windowID != SDL_GetWindowID(ctx->window))
             return true;
 
+        // Never nest NewFrame() inside an already-open frame — Windows min/max/restore buttons
+        // (renderBorderlessControls) push SDL window events synchronously from inside app.render(). (C146)
+        ImGuiContext* gctx = ImGui::GetCurrentContext();
+        if (!gctx || gctx->WithinFrameScope) return true;
+
+        // Dedupe the RESIZED + PIXEL_SIZE_CHANGED pair: both report the same final pixel size, so render
+        // once. SDL_GetWindowSizeInPixels is a consistent key across both event types (their data1/data2
+        // differ — points vs pixels — and cannot be compared directly).
+        int pw = 0, ph = 0;
+        SDL_GetWindowSizeInPixels(ctx->window, &pw, &ph);
+        static int lastW = 0, lastH = 0;
+        if (pw == lastW && ph == lastH) return true;
+        lastW = pw; lastH = ph;
+
         // Avoid re-entrancy
         static bool in_resize_render = false;
         if (in_resize_render) return true;