ifcviewer: move configureSurface into ViewportCore (#84-u)

The swapchain configuration path — WGPU_PRESENT_MODE handling,
mode-preference order (Mailbox / Immediate / FifoRelaxed / Fifo),
caps probe, cfg.alphaMode wiring, and the on-resize depth + MSAA +
HiZ texture reallocation + bind-group invalidation — all move to
ViewportCore. The only VW-side concern was a QString::fromLatin1 in
the advertised-modes log, which becomes a std::string concat with the
same output.

ViewportWindow's render() / resizeEvent / surface-outdated retry paths
call core_.configureSurface() now. createSurface() (which returns the
platform-specific WGPUSurface from the host) stays VW because Qt
platform discovery has to happen on the Qt side.

src/ifcviewer/ViewportWindow.h

@@ -264,7 +264,7 @@ protected:
 private:
     bool initWgpu();
     bool createSurface();
-    void configureSurface(int width_px, int height_px);
+    // configureSurface moved to ViewportCore (#84-u).
     void render();
     void shutdown();