Validate same-block spend balances

[webbananaunite]

Summary

• Include already-validated same-block transactions when checking account balance.
• Validate received blocks sequentially so multiple spends from the same balance are aggregated.
• Preserve debit, credit, and account fields when serializing and restoring transactions for block validation.
• Add regression tests for rejecting same-block overspending and allowing spends within balance.

Verification

• swift test --build-path /Users/yoichi/appOutput/blocks
• /Users/yoichi/.swiftly/bin/swift build -v --swift-sdk x86_64-swift-linux-musl --build-path /Users/yoichi/appOutput/blocks-linux-musl