UX enhancements for a chromeless UI on Android

A functional player still needs clear feedback and convenient interactions. Plan these experiences alongside the core playback controls:

1. Loading spinner: Indicate when the player is waiting for video data.
2. Poster: Show an image before initial playback and after the video ends.
3. Up next: Offer the next stream near the end of playback and optionally start it automatically.
4. Skip intro: Let viewers skip the current introduction.
5. Ad countdown: Display the remaining duration of an ad break.
6. Ad markers: Indicate ad-break positions on the scrub bar.

Drive each enhancement from Player events and state. Start with tracking player states, then account for Android lifecycle transitions and configuration changes.