[codex] Add shared playback tests and CI

[moonggae]

Summary

• Extract playlist and shuffle behavior into common, testable state objects.
• Fix iOS removal handling so deleting a non-current item preserves the current track.
• Make the sleep timer testable with an internal clock abstraction and deterministic tests.
• Extract Android playback command routing behind an internal command target and test command order plus volume scaling.
• Add a GitHub Actions workflow for shared KMP checks and Android shared target compilation.

Validation

• ./gradlew :shared:check --console=plain
• ANDROID_HOME=/Users/choi/Library/Android/sdk ./gradlew :shared:compileAndroidMain --console=plain
• ./gradlew :kmedia-sample-android:assembleDebug --console=plain