Run the same-device Vulkan OpenXR backend on desktop Linux

Lepton's Vulkan driver has no external memory or sync fd extensions,
so the Steam Frame gets a native SteamOS build instead. Its backend is
the PC's: OpenXR creates Dawn's own Vulkan instance and device, and the
eyes are copied into the swapchain on Dawn's queue, with nothing shared
between devices.

- openxr_vulkan_win32 and Aurora's vulkan_win32_interop now compile on
  desktop Linux. Dawn links statically there, so the bridge calls the
  patched package's hooks directly, and exists only when the package's
  aurora-dawn.json declares the Vulkan hook ABI (AuroraDawnProvider
  turns AuroraVulkanAbi into AURORA_DAWN_VULKAN_HOOKS); a stock Dawn
  keeps the stubs and VR falls back to the desktop.
- openxr_integration.cpp gains a Linux branch: XR_KHR_vulkan_enable2,
  the timespec clock, refresh rate, performance level, the Frame
  controllers and eye gaze; fragment density maps are requested on
  Linux as on the Quest (fdm.cpp and gpu.cpp, AURORA_FDM=0/1 overrides).
- Controller motion uses XR_KHR_convert_timespec_time on any Linux.
- CMake: Vulkan headers are fetched for Linux OpenXR builds, the
  headset option is MKW_HEADSET on Android and Linux, and Linux aarch64
  builds take MKW_LINUX_CPU (cortex-x4 for the Steam Frame, else native).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

aurora-main/lib/webgpu/gpu.cpp

@@ -830,6 +830,13 @@ bool initialize(AuroraBackend auroraBackend) {
     const int fdmOverride = android_debug::property_int("debug.wiicompiled.fdm", -1);
     fdm::request(g_backendType == wgpu::BackendType::Vulkan &&
                  (fdmOverride >= 0 ? fdmOverride == 1 : g_config.xrFragmentDensityMap));
+#elif defined(__linux__)
+    // Desktop Linux (SteamOS on the Steam Frame), with a Dawn built by Launcher/build-dawn-linux.sh.
+    // AURORA_FDM=0 or 1 overrides the settings, as debug.wiicompiled.fdm does on the Quest.
+    const char* fdmOverride = std::getenv("AURORA_FDM");
+    fdm::request(g_backendType == wgpu::BackendType::Vulkan &&
+                 (fdmOverride != nullptr && *fdmOverride != '\0' ? std::strcmp(fdmOverride, "1") == 0
+                                                                 : g_config.xrFragmentDensityMap));
 #endif
     const auto future =
         g_adapter.RequestDevice(&deviceDescriptor, wgpu::CallbackMode::WaitAnyOnly,