feat: add opt-in video caching for Android and iOS

[kdroidFilter]

Summary

• Add CacheConfig data class (opt-in, disabled by default) with configurable max cache size
• Android: shared SimpleCache singleton with LeastRecentlyUsedCacheEvictor, injected via CacheDataSource.Factory into ExoPlayer's DefaultMediaSourceFactory
• iOS: configure NSURLCache with increased disk capacity so AVPlayer's HTTP responses are cached transparently
• JVM/Web: accept CacheConfig parameter (no-op)
• Add clearCache() to VideoPlayerState interface (default no-op, implemented on Android & iOS)
• Add media3-datasource and media3-database explicit dependencies

Usage

val playerState = rememberVideoPlayerState(
    cacheConfig = CacheConfig(enabled = true, maxCacheSizeBytes = 200L * 1024 * 1024)
)
playerState.openUri("https://example.com/video.mp4") // cached on disk

Closes #34

Test plan

• Android: verify video plays normally with CacheConfig(enabled = false) (default)
• Android: enable cache, play a video, kill the player, replay the same URL — should load from cache (check logcat for cache hits)
• Android: verify clearCache() removes cached data
• iOS: same flow — enable cache, play, replay, verify faster load
• iOS: verify clearCache() clears NSURLCache
• JVM/Web: verify no compilation errors, config is ignored gracefully
• Verify existing tests still pass on all targets