fix(queue): prevent duplicate message send (race condition)

[Yuu6798]

🐛 Problem

Concurrent workers can enqueue identical messages, producing duplicate outputs and potential double-billing.

🔧 Fix

Adds DuplicateGuardQueue with

1. SHA-256 hash deduplication
2. mutex to guarantee single enqueue

✅ Test

pytest -q test_duplicate_send.py → green (1 passed)

📄 Files

• queue_stub.py → minimal guard implementation
• test_duplicate_send.py → multithread regression test
• project_types.py → rename to avoid stdlib clash

Closes: #