Closes #2268

Fixes ownership issue in send_tx where the Queue struct was moved
into the first thread closure, preventing the second thread from accessing it.

Changes:

• Destructured Queue into first_half and second_half
• Cloned mpsc::Sender so both threads can send messages concurrently

All tests pass locally:

rustc --test exercises/20_threads/threads3.rs && ./threads3

=> test result: ok. 1 passed; 0 failed