Show the headset settings panel as its own OpenXR quad layer

The panel was drawn into the eye images, so below a 1.00x OpenXR
resolution its 1440x1080 canvas was minified into a small eye region and
the text became hard to read. It is now submitted as a quad layer of its
own over the scene's projection or menu quad, which the compositor samples
directly at any render scale.

Each backend (D3D12, Windows Vulkan, Quest) makes the panel's swapchain
pair (plus two shared buffers on the Quest) the first time the panel
opens. While it is open, the frame hands Aurora one more stereo target
after the eyes, which the bridge fills with the panel texture or a
transparent image. The panel image follows the eyes' displayed/retained
pairing, so a cancelled frame never shows an unwritten panel. The quad
hangs where the pointer's hits are tested. Aurora leaves the panel out of
the eyes in layer mode, and a backend that cannot make the layer falls
back to drawing it into the eyes.

On the Quest, debug.wiicompiled.panel_layer 0 selects the old path at
run time. Measured there at a race start (render_scale 0.8), the layer
costs nothing while the panel is closed; while it is open, app GPU time is
10.5 ms against 9.7 ms drawn into the eyes, with unchanged frame rates.

The replay tests cover lazy creation, cancelled frames, closed and
unplaced panels, and render-first pacing on D3D12 and Windows Vulkan.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

aurora-main/lib/stereo_overlay.hpp

@@ -3,6 +3,8 @@
 #include <aurora/math.hpp>
 #include <webgpu/webgpu_cpp.h>
 
+#include "stereo.hpp"
+
 #include <cstdint>
 
 struct ImDrawData;
@@ -28,6 +30,20 @@ void composite_immersive(const wgpu::CommandEncoder& encoder, const wgpu::Textur
 void composite_flat(const wgpu::CommandEncoder& encoder, const wgpu::TextureView& eye, const wgpu::Extent3D& size,
                     uint32_t eyeIndex) noexcept;
 
+// Layer mode: the OpenXR backend shows the panel as its own compositor quad
+// layer, sharp at any eye resolution, so it is no longer drawn into the eyes
+// (both composite functions do nothing). Any thread; read by the frame worker.
+void set_layer_mode(bool enabled) noexcept;
+bool layer_mode() noexcept;
+
+// Frame worker, inside a stereo sink: the image to copy into a panel layer of
+// width x height in the eyes' format. That is the panel while it is showing at
+// exactly that size, and otherwise a transparent image of that size, so a layer
+// asked for before the panel's first frame (or after it closed) shows nothing.
+// A transparent image is cleared once, by a pass recorded into `encoder`.
+// False only when no image can be made.
+bool layer_source(const wgpu::CommandEncoder& encoder, uint32_t width, uint32_t height, stereo::EyeImage& out) noexcept;
+
 void shutdown() noexcept;
 
 } // namespace aurora::stereo_overlay