[byungjul]

Fixes the session-stop / quit crash reported as DisplayXR/displayxr-runtime#1779. The cause is in the plugin, not the runtime.

Problem

On Vulkan, every session stop (XRGeneralSettings.Instance.Manager.StopSubsystems()) and every app quit crashed with SIGSEGV. That made a session restart impossible on Linux.

A core dump against a runtime built with symbols shows two threads at the crash:

runtime repaint thread (crashing):
  vkQueuePresentKHR (libnvidia-glcore)
  comp_vk_native_target_present      comp_vk_native_target.cpp:3546
  vk_dp_weave_and_present            comp_vk_native_compositor.c:6392
  vk_repaint_thread                  comp_vk_native_compositor.c:7190

plugin, at the same moment:
  vkDestroyInstance
  dxr_pvk_destroy
  dxr_prov_session_stop
  GfxStop

dxr_prov_session_stop called dxr_pvk_destroy() before xrEndSession, and dxr_pvk_destroy() destroyed the VkDevice and VkInstance that we created through XR_KHR_vulkan_enable2. The session still runs on them. The runtime's repaint thread keeps presenting the last frame until xrEndSession, so it presented on a destroyed device.

Fix

The teardown is split in two:

• dxr_pvk_destroy() destroys the bridges, overlays, fence and command pool. It still runs before xrEndSession.
• dxr_pvk_destroy_device() is new. It destroys the device and instance after xrDestroySession and xrDestroyInstance.

Platforms: Vulkan backend only (ENABLE_VULKAN): Linux, and Windows with DISPLAYXR_VK_EXPERIMENTAL=1. No change on D3D11/D3D12/Metal.

Testing

Ubuntu 26.04, RTX 4090 Laptop, Leia DS1, installed runtime v2.21.11. App: the lenovo-avatar Linux player.

Test	Before	After
Quit (×3)	SIGSEGV every time	No crash, destroyed logged after the session
Session restart every 8 s for 50 s	Crash on the first stop	4 restarts, each back to FOCUSED; no xrEndFrame error, no Xid
Session restart every 12 s with both HUDs and the Local2D bubble on	—	4 restarts; each new session recreates the Local2D layer (session token 1 → 5) and both wsui slots, and the first Unity copy is recorded for all three. No errors

The last row is the restart check requested on #337 for the Local2D/wsui Vulkan paths.

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