skill: add Android skills

.agents/skills/camerax/references/xr.md

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+Developing camera features for XR devices, headsets, and AR glasses requires a
+shift from 2D pixel-pushing to 3D spatial awareness.
+
+## Understand the XR development mindset
+
+| Concept | Mobile focus | **XR focus** |
+|---|---|---|
+| **Input** | Raw camera stream | **Spatial tracking (visual-inertial odometry (VIO) or simultaneous localization and mapping (SLAM))** |
+| **Output** | Screen viewfinder | **Stereo passthrough and occlusion** |
+| **Constraint** | Battery life | **Motion-to-photon latency (less than 20 ms)** |
+
+*** ** * ** ***
+
+## Follow the implementation guide
+
+### API selection
+
+On XR devices, standard `CameraX` implementations are often restricted or
+insufficient. Always use spatial software development kits (SDKs):
+
+- **ARCore**: Use ARCore for plane detection, depth sensing, and motion tracking.
+- **OpenXR**: Use OpenXR as the cross-platform standard for VR and AR rendering and input.
+- **OEM SDKs**: Use manufacturer-specific libraries for hardware-accelerated passthrough.
+
+### Handle spatial passthrough
+
+Unlike a 2D viewport, XR passthrough is often system-managed.
+
+**\[Key requirement\] Frame synchronization**: Synchronize your application's
+frame clock with the headset's head-mounted display (HMD) pose.
+
+```kotlin
+// Example: Querying the spatial pose for the current camera frame
+val headPose = xrSession.getHeadPose(frameTime)
+val projectionMatrix = headPose.getProjectionMatrix(eyeIndex)
+```
+
+<br />
+
+### Manage depth and occlusion
+
+Digital content must respect real-world depth to ensure accurate occlusion.
+
+- **Depth map** : Access raw depth data using `ARCore` or `SurfaceProcessor` to create an occlusion mask.
+- **Hardware buffers** : Use `HardwareBuffer` to share camera frames directly with the GPU without CPU-side copies to minimize latency.
+
+*** ** * ** ***
+
+## XR pitfalls
+
+- **The nausea limit**: Any processing that delays the viewfinder by more than 20 ms causes user sickness. Don't perform image processing on the main thread.
+- **Privacy restrictions** : Some XR devices return a black frame if you attempt to record the "Passthrough" layer. Check `Session.isRecordingSupported`.
+- **Field of view (FOV)**: The camera FOV possibly doesn't match the display FOV. Use the SDK's projection matrixes instead of calculating aspect ratios manually.
+- **Front buffer rendering** : If the device supports it, use `FrontBufferRenderer` for real-time overlays to bypass standard double-buffering latency.