skill: add Android skills

.agents/skills/camerax/references/testing.md

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+Automated testing for camera features is notoriously difficult because you
+can't easily mock physical hardware, lighting, or motion. This guide provides
+patterns for reliable, hermetic camera tests.
+
+## Develop a testing mindset
+
+| Challenge | Conventional approach | Camera technical approach |
+|---|---|---|
+| **Frameworks** | `Mockito` or `MockK` | **Fakes over mocks** |
+| **Assertions** | `assertEquals`, `assertTrue` | **Google Truth, `assertThat`** |
+| **Environment** | Implied environments | **Explicit `@RunWith` annotations** |
+| **Async operations** | `Thread.sleep` | **Explicit `timeoutMillis` or `IdlingResource`** |
+
+*** ** * ** ***
+
+## Follow the implementation guide
+
+### Fakes over mocks
+
+**Don't use Mockito.** Relying on mocks for complex, rapidly changing interfaces
+like `ImageProxy` or `CameraInfo` makes tests brittle. Instead, build "Fake"
+implementations that verify state rather than behavior.
+
+
+```kotlin
+// Create a Fake ImageProxy for ML Testing (Fakes over Mocks)
+val fakeImage = FakeImageProxy(w = 640, h = 480)
+
+// Feed the fake buffer into your analyzer
+```
+
+<br />
+
+### Mock camera capabilities
+
+Use `FakeAppConfig` from `androidx.camera:camera-testing` to simulate specific
+hardware constraints in tests, such as a device without a flash.
+
+
+```kotlin
+// Use awaitInstance() extension function for coroutine-based provider retrieval
+val cameraProvider = ProcessCameraProvider.awaitInstance(context)
+```
+
+<br />
+
+### Use Truth assertions
+
+Use Google Truth, `assertThat`, instead of standard JUnit assertions. It
+provides more readable assertion chains and useful failure messages.
+
+### Test asynchronous lifecycles
+
+Camera initialization is asynchronous. Use `IdlingResource` to ensure
+your test waits for the `UseCase` to be bound before asserting.
+
+To test asynchronous lifecycles, use the following pattern:
+
+1. Wrap the `ProcessCameraProvider` initialization in a `CountDownLatch` or `IdlingResource`.
+2. Assert only after the `cameraControl` instance is non-null.
+
+*** ** * ** ***
+
+## Testing pitfalls
+
+- **Resource leaks** : To prevent "Camera in Use" errors, in your `@After` block, call `cameraProvider.unbindAll`.
+- **The "flaky initializer"**: Camera tests often fail on CI because the "Virtual Camera" takes too long to warm up. Use a sufficient explicit timeout for the first initialization.
+- **Permission blockers** : To bypass the system permission dialogs, in your Espresso tests, use `GrantPermissionRule`.
+- **Resolution mismatch** : Tests on emulators often default to 640x480. Ensure your `ResolutionSelector` handles this low-res fallback correctly.