feat(chain): add parallel chain branches

[maycon]

Summary

Add support for parallel execution branches within a chain. This enables testing complex scenarios where multiple actions must happen simultaneously after setup steps.

Parent Issue

• Part of feat: Add request chaining support for race conditions #30
• Depends on feat(chain): add conditional execution and sync points #32

Configuration

race_attack:
  race:
    threads: 20
    
    chain:
      # Sequential setup
      - name: login
        request: "POST /login"
        extract:
          session: "cookie.session"
      
      - name: get_balance
        request: "GET /api/balance"
        extract:
          balance: "json.amount"
      
      # Parallel branches - execute simultaneously
      - parallel:
          - name: withdraw_cash
            request: "POST /api/withdraw"
          
          - name: transfer_out
            request: "POST /api/transfer"
          
          - name: buy_item
            request: "POST /api/purchase"
      
      # Continue sequentially after parallel completes
      - name: verify
        request: "GET /api/balance"

Execution Flow

Sequential:    [login] → [get_balance] → ...
                                         ↓
Parallel:                          ┌─[withdraw]─┐
                                   ├─[transfer]─┤ (all 3 run simultaneously)
                                   └─[purchase]─┘
                                         ↓
Sequential:                          [verify]

Use Cases

1. Multi-Vector Attack

parallel:
  - name: drain_balance
    request: "POST /api/withdraw?amount=all"
  
  - name: request_refund
    request: "POST /api/refund?order=123"
  
  - name: transfer_to_friend
    request: "POST /api/transfer?to=friend"

2. Resource Exhaustion

parallel:
  - name: book_seat_1
    request: "POST /api/book?seat=1A"
  
  - name: book_seat_2
    request: "POST /api/book?seat=1B"
  
  - name: book_seat_3
    request: "POST /api/book?seat=1C"

Implementation Notes

• Parallel branches execute in separate threads/coroutines
• All branches must complete before chain continues
• Each branch has access to same chain context (read-only for parallel section)
• Results from parallel branches merged into chain context

Acceptance Criteria

• Parse parallel blocks in chain config
• Execute parallel steps concurrently
• Wait for all parallel steps to complete
• Merge parallel results into chain context
• Handle errors in parallel branches
• Support nested parallel blocks (optional)
• Unit tests
• Documentation

Future Consideration

This is an advanced feature that may be implemented in a future version after basic chaining is stable.