Fixed

• Spigot compatibility – ItemStack data is now stored as a Base64-encoded byte stream (item-data key) via BukkitObjectOutputStream instead of YAML-native serialization. This eliminates the [ERROR] Material cannot be null log entries that appeared during startup when loading YAML files written by a Paper 1.21+ server on a Spigot server. Existing files are migrated to the new format automatically on the next save cycle; the old item key is still read as a fallback during the transition.
• Smoke test coverage – Added YamlAuctionStorageSmokeTest (8 tests) and a dedicated spigot-smoke.yml GitHub Actions workflow to guard this code path in CI.