v0.4.0: Direct3D 11/12 backends, in-game renderer selection, per-run logging

- ra3.dx: D3D11 and D3D12 presentation backends (runtime HLSL via d3dcompiler);
  2D image blit and the GPU heightfield terrain raymarch, with the corner
  minimap/FPS overlays. Non-Windows builds link a null fallback.
- ra3.display: preferred backend plus ordered fallback (Vulkan/D3D11/D3D12/SDL);
  menu gains a Renderer option and the CLI gains --dx11/--dx12/--sdl, which the
  render command now honours too.
- vendor libenderlog (MIT): every run writes openra3.log next to the exe and
  archives the previous run's log; records at warn and above carry a call stack
  (native fallback, since libc++ has no <stacktrace>).
- Windows crash reporter writes openra3_crash.log (faulting module + backtrace);
  D3D/DXGI diagnostics are routed through the logger.

src/ra3.cppm

@@ -18,4 +18,5 @@ export import ra3.render;
 export import ra3.terrain;
 export import ra3.ui;
 export import ra3.vulkan;
+export import ra3.dx;
 export import ra3.display;