Steam Frame native build: patched Dawn for Linux, local-build VR options, docs

Launcher/build-dawn-linux.sh builds the pinned Dawn with Aurora's Vulkan hook
and density map patches into a package local-build.sh takes through the new
--dawn-package option, together with --openxr, --headset steam_frame and
--cpu. docs/steam-frame.md now leads with the native SteamOS build (building
in a podman container on the Frame, running it, the log lines to expect);
OPENXR.md's backend table records the Linux Vulkan backend and why the Lepton
flavour cannot present.

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

OPENXR.md

@@ -1276,8 +1276,8 @@ custom DLL's ABI through three borrowed-image copy/readback cycles; run it with
 | Windows D3D12 | Implemented: same-adapter, same-device asynchronous OpenXR submission. |
 | Windows Vulkan | Implemented, opt-in (`video.graphics_api = "vulkan"`): the runtime creates Dawn's Vulkan instance and device through `XR_KHR_vulkan_enable2`, eyes are copied on the same queue, and Dawn's device guard is held around the four queue-touching OpenXR calls. Needs the custom Dawn from `Launcher/Build-DawnVulkan.ps1`. Raced on SteamVR/PSVR2 at the headset's full rate; other runtimes unexercised. See [Windows Vulkan](#windows-vulkan). |
 | Android Vulkan (Meta Quest) | Implemented and running on a Quest 3: the OpenXR side owns its own Vulkan device (`XR_KHR_vulkan_enable2`, `XR_KHR_vulkan_enable` fallback) and shares eyes with Dawn through `AHardwareBuffer`s ordered by sync-fd fences. Controllers arrive through OpenXR actions as a virtual SDL gamepad. See `docs/quest-port.md`. |
-| Android Vulkan (Steam Frame) | The same backend in the `steamFrame` flavour, for SteamVR's Android runtime under Lepton: Frame controller profile, 120 Hz request, eye-tracked foveation. Built and unit-tested, not yet run on the headset. See `docs/steam-frame.md`. |
-| Linux Vulkan | Not wired. The pinned Dawn package does not expose a native Vulkan device, and the AHardwareBuffer bridge is Android-only; a dma-buf/opaque-fd variant of the same design would cover desktop Linux. |
+| Android Vulkan (Steam Frame) | The same backend in the `steamFrame` flavour, for SteamVR's Android runtime under Lepton: Frame controller profile, 120 Hz request, eye-tracked foveation. Blocked on the headset: Lepton's Turnip exposes no AHardwareBuffer or fd external memory, so the eyes cannot cross devices. The native build below is the supported route. See `docs/steam-frame.md`. |
+| Linux Vulkan (Steam Frame, SteamOS) | Implemented, not yet run on the headset: the Windows Vulkan design compiled for Linux, so the runtime creates Dawn's own instance and device and the eyes are copied on Dawn's queue with no sharing. Adds the Frame controller profile, 120 Hz and gaze-centred foveation. Needs a Dawn built with Aurora's patches (`Launcher/build-dawn-linux.sh`), then `Launcher/local-build.sh --openxr --dawn-package <dir> --headset steam_frame`. See `docs/steam-frame.md`. |
 | Other platforms | Not wired yet. |
 
 Both bindings share `openxr_integration.cpp`: the pacing thread, policy evaluation, the