skill: add Android skills

.agents/skills/camerax/references/foldables.md

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+Foldable devices introduce unique challenges for camera applications, including
+dynamic layout changes, multiple display orientations, and physical device
+postures, such as tabletop and book modes.
+
+## Manage fold states and postures
+
+| State | Posture | User interaction | Implementation goal |
+|---|---|---|---|
+| `FLAT` | Standard | Full screen preview | Conventional mobile phone layout. |
+| `HALF_OPENED` | Tabletop | Lower half for controls | Split-screen layout, viewfinder on top, controls on bottom. |
+| `HALF_OPENED` | Book | Side-by-side | Viewfinder on one panel, gallery and controls on the other. |
+
+*** ** * ** ***
+
+## Follow the implementation guide
+
+### Detect posture changes
+
+Use the Jetpack WindowManager library to observe the device's hinge state and
+fold layout.
+
+
+```kotlin
+lifecycleScope.launch {
+    lifecycle.repeatOnLifecycle(Lifecycle.State.STARTED) {
+        windowInfoTracker.windowLayoutInfo(activity)
+            .collect { layoutInfo ->
+                val displayFeature = layoutInfo.displayFeatures
+                    .filterIsInstance<FoldingFeature>()
+                    .firstOrNull()
+
+                updateCameraLayout(displayFeature)
+            }
+    }
+}
+```
+
+<br />
+
+### Handle tabletop mode
+
+In Tabletop mode, horizontal fold, you should move the viewfinder to the top
+half of the screen and the controls to the bottom half to prevent the user from
+seeing a "bent" image.
+
+- **Identify orientation:** Check `FoldingFeature.orientation`.
+- **Calculate geometry:** Use `FoldingFeature.bounds` to identify the hinge's physical location on the screen.
+- **Update UI:** Apply padding or constraints to move the `PreviewView` above the hinge.
+
+### Coordinate mapping and `Viewport`
+
+When the UI layout changes due to a fold, you **must** update the `Viewport` to
+ensure that tap-to-focus and image capture coordinates remain accurate.
+
+
+```kotlin
+val viewport = ViewPort.Builder(Rational(viewfinder.width, viewfinder.height), display.rotation)
+    .setScaleType(ViewPort.FILL_CENTER)
+    .build()
+
+val useCaseGroup = UseCaseGroup.Builder()
+    .addUseCase(preview)
+    .setViewPort(viewport)
+    .build()
+```
+
+<br />
+
+### Rear display mode
+
+Some foldables allow using the rear camera with the cover display while the
+device is unfolded.
+
+- **Verification:** If available through OEM SDKs or Android 14 (API level 34) or higher, check `DeviceState.REAR_DISPLAY_STATE`.
+- **Logic:** Handle preview detachment and reattachment on different display surfaces with varying aspect ratios.
+
+*** ** * ** ***
+
+## Foldable pitfalls
+
+- **Hinge distortion:** Don't span the camera preview across a hinge in `HALF_OPENED` state.
+- **Physical orientation:** The camera sensor's physical orientation relative to the screen often changes when you fold or unfold the device. Always rely on `CameraInfo.getSensorRotationDegrees`.
+- **Latency:** Rebinding `UseCase` objects during a fold event is expensive. Use the internal scaling of `PreviewView` before performing a full `bindToLifecycle` reconfiguration.