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

runtime/cmake/PublicProducts.cmake

@@ -452,7 +452,14 @@ elseif(MKW_PLATFORM_ANDROID)
     set(MKW_ANDROID_CPU "cortex-a77" CACHE STRING "AArch64 -mcpu target for the Android products")
     set(MKW_BASELINE_ARCH_FLAG -mcpu=${MKW_ANDROID_CPU})
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64|ARM64)$")
-    set(MKW_BASELINE_ARCH_FLAG -mcpu=native)
+    # A cross-build names its CPU; the Steam Frame's Snapdragon 8 Gen 3 exposes SVE (cortex-x4).
+    if(MKW_HEADSET STREQUAL "steam_frame")
+        set(_mkw_linux_cpu_default "cortex-x4")
+    else()
+        set(_mkw_linux_cpu_default "native")
+    endif()
+    set(MKW_LINUX_CPU "${_mkw_linux_cpu_default}" CACHE STRING "AArch64 -mcpu target for the Linux products")
+    set(MKW_BASELINE_ARCH_FLAG -mcpu=${MKW_LINUX_CPU})
 else()
     set(MKW_BASELINE_ARCH_FLAG "")
 endif()