fix(vidstack): preload adjacent thumbnail sprites

[mihar-22]

Summary

• Preload thumbnails in a bounded window of the next two entries after the active thumbnail.
• Skip entries that are still on the active sprite while relying on the preload cache to dedupe repeated future sprite URLs.
• Avoid walking the entire future sprite list into memory while still warming adjacent seek targets.
• Key preload cache entries by sprite URL and crossOrigin, with a bounded LRU cache.
• Add focused regression coverage for bounded adjacent sprite preloading.

Fixes #1653.

Coordination

This should merge after #1825 because both PRs touch thumbnail.ts and thumbnail tests. A dry merge check against #1825 head completed without conflicts.

Validation

• pnpm --filter vidstack exec vitest --run src/components/ui/thumbnails/thumbnail-preload.test.ts
• pnpm --filter vidstack typecheck
• git diff --check