Add comprehensive idle downloader stress tests

- Add offset-based deterministic test data generation for data integrity verification
- Each 8 bytes contains big-endian offset, allowing corruption detection
- TestIdleDownloadIntegrity: verifies idle downloads produce correct data
- TestIdleDownloadWithConcurrentReads: concurrent reads during idle downloading
- TestIdleDownloadDoesNotBlockReads: verifies reads aren't blocked by idle tasks
- TestMultipleReadersIntegrity: ZIP-like random access pattern testing
- TestIdleDownloadStress: heavy concurrent access stress test
- TestIdleDownloadCompletion: verifies idle download completes files

Coverage improved from 67.8% to 76.3%

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MagicalTux