Implement task assignment logic in coordinator after `TaskClaim`

[iTranscend]

When a TaskClaim is received, the coordinator only prints "Task claim received!!" to stdout. There is no:

• Deduplication of multiple claims for the same task
• Worker selection strategy (first-come, lowest estimated duration, capability match)
• TaskAssignment message sent back to the selected worker
• Task status update from Pending → Running

Location

src/node.rs, lines 500–503

Message::TaskClaim(task_claim) => {
    task_claim.validate()?;
    println!("Task claim received!!");
}

Suggested approach

1. Add a TaskAssignment message variant to the Message enum.
2. Track pending claims per TaskId in the coordinator.
3. After a configurable claim window, select a worker and send TaskAssignment.
4. Update TaskStatus accordingly.