[v1.2] FIX 4.4 session layer

## Task
Implement `services/shared/marketdata/collectors/fix_collector.py`.

## Acceptance
- [ ] FIX 4.4 Logon/Logout/Heartbeat
- [ ] NewOrderSingle, ExecutionReport parsing
- [ ] ResendRequest on gap
- [ ] Unit tests for message construction

## Notes
Target institutions with DMA. Use `fixlib` or hand-rolled.