Add LazyColumn shortform demo with single-player preloading (addresses #1853)

[Amud]

MicrosoftTeams-video.mp4

Adds a LazyColumn-based shortform demo that optimizes a single-ExoPlayer setup with preloading, addressing performance concerns raised in #1853.

Related issue

Fixes #1853.

Background

• Initial frame delays with single player + PreloadManager
• Lag on first video and during transitions
• Performance gap vs a multi-player pool approach

Solution

• Window-based media management
  • Sliding window (~10 items) with dynamic load/unload based on scroll
  • Reduces memory pressure while maintaining smooth playback
• Optimized preloading
  • TargetPreloadStatusControl preloads 3–5s for adjacent items
  • Preloads items at distance ±1 and ±2
• Reactive cache tracking
  • mutableStateMapOf tracks cache completion
  • Thumbnails load only after content is cached
• Single player benefits
  • Lower memory use than a 3‑player pool
  • Simpler state management with smooth transitions

Performance (observed)

• Reduced initial frame delay via targeted preloading
• Windowing avoids memory exhaustion
• Cache-aware thumbnails improve perceived responsiveness
• Consistent transition smoothness across indices

Implementation highlights

• LazyColumnPlayerManager: sliding window + preload management
• LazyColumnTargetPreloadStatusControl: 3–5s buffer strategy
• SinglePlayerSetupHelper: optimized load control and cache
• Compose UI reacts to cache status changes

Testing

• Transitions across all items
• Smooth playback without noticeable frame drops
• Stable memory during extended scrolling
• No rendering delays after initial cache warm-up

Entry point

Adds a “LazyColumn” button in MainActivity alongside the existing ViewPager demo.