Add reliable Item Stash delivery

[ImCZFy]

Summary

• add an optional ServicesManager-based Skyblock-Core item-delivery hook with an atomic inventory-only fallback
• route BIN purchases, cancellations, expired returns, winning bids, and administrator returns through reliable escrow-preserving delivery
• add stable operation IDs, durable purchase settlements, crash recovery, Vault uncertainty handling, and GUI click locks
• add configurable integration settings and delivery messages
• keep the Skyblock-Core API compile-only and fail packaging if any API class leaks into the plugin JAR

Verification

• ./gradlew clean test shadowJar
• 27 tests passed, 0 skipped
• packaged JAR contains 0 dev/chengzhi/skyblockcore classes
• YAML and UTF-8 message resources validated
• no server was started or reloaded