Fix selected YouTube caption translation not appearing

[wizdom13]

• Build the currently selected YouTube caption translation again at playback-resolution time so cached StreamInfo cannot hide a newly selected translation language.
• Reuse an extracted native/manual caption URL when possible, with auto-generated captions as the fallback source.
• Avoid adding a duplicate when the requested language already exists as a native or translated track.
• Keep the earlier extractor-time translation path and the Media3 TTML decoding fix intact.
• Add unit coverage for cached Arabic-to-English translation generation and playback-resolver integration.
• Update the 1.15.0 Fastlane changelog to mention the selected-track fix while keeping it ASCII-only and 459 bytes for IzzyOnDroid compatibility.