skill: add Android skills

.agents/skills/camerax/references/mlkit-spatial.md

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+When you use ML Kit for features such as face mesh, object detection, or pose
+detection, the most common failure point is the coordinate disparity between the
+analysis image and the viewfinder UI.
+
+## The mapping mindset
+
+| Dimension | Analysis frame | Viewfinder UI on the screen |
+|---|---|---|
+| Resolution | Fixed, such as 640x480 | Dynamic, such as 1080x2400 |
+| Rotation | 0° for the raw buffer | 90° or 270° in portrait or landscape mode |
+| Origin | Top-left of buffer at (0,0) | Top-left of screen at (0,0) |
+
+*** ** * ** ***
+
+## Follow the implementation guide
+
+### Coordinate transformation matrix
+
+Android provides the `Viewport` and `UseCaseGroup` APIs to calculate the
+transformation matrix automatically. **Don't** calculate aspect ratio scaling
+manually.
+
+
+```kotlin
+val transform = previewView.viewPort?.let { viewPort ->
+    // Use CameraX's built-in coordinate mapper
+    viewPort.getTransformationMatrix(imageProxy.imageInfo.rotationDegrees)
+}
+```
+
+<br />
+
+### Handling the "double rotation" bug
+
+ML Kit results, bounding boxes, are relative to the **rotated buffer**. If the
+device is in portrait, the buffer is often 480x640, landscape, but the screen
+is 1080x1920.
+
+To map the coordinates, use the following workflow:
+
+1. Query `imageProxy.imageInfo.rotationDegrees`.
+2. Pass this rotation to the ML Kit `InputImage`.
+3. Use the `MappingUtils.transformRect` method to map the result `Rect` to the screen.
+
+### Face mesh and pose normalization
+
+For high-precision spatial analysis, for example, "Is the user's hand at a
+specific screen button?", use **normalized coordinates from 0.0 to 1.0**.
+
+
+```kotlin
+// Example: Converting a Pose landmark to a Screen Coordinate
+val screenX = landmark.position.x / analysisWidth * screenWidth
+val screenY = landmark.position.y / analysisHeight * screenHeight
+```
+
+<br />
+
+**Warning** : Always account for **mirrored lenses** . If the `LENS_FACING_FRONT`
+is used, you must flip the X-coordinate: `actualX = screenWidth - screenX`.
+
+### Overlays and canvas clipping
+
+Use a custom `GraphicOverlay` view on top of the `PreviewView`.
+
+- **Buffer lock** : Ensure your `GraphicOverlay` clears its canvas every time a new `ImageAnalysis` frame is processed to prevent "ghosting" of bounding boxes.
+
+*** ** * ** ***
+
+## Spatial pitfalls
+
+- **The "stretched box" bug** : Caused by assuming the Analysis Frame aspect ratio, 4:3, matches the screen aspect ratio, 21:9. Use `PreviewView.SCALE_TYPE_FILL_CENTER` and map coordinates accordingly.
+- **Latency** : If ML processing exceeds 50 ms, the bounding box trails behind the user's face.
+  - **Fix** : Use `ImageAnalysis.STRATEGY_KEEP_ONLY_LATEST` to avoid queuing stale frames.
+- **Sensor vs. display rotation** : On some tablets, the sensor is mounted horizontally. Always query `display.rotation` and `cameraInfo.sensorRotationDegrees`.