Fix IBKR headless gateway startup

[Pigbibi]

Summary

• restore upstream IBC session handling defaults in docker-compose
• override the upstream headless run script to use a 24-bit Xvfb display and x11vnc with -noxdamage
• document the new defaults and add compose/startup regression checks

Validation

• bash tests/test_docker_compose_ports.sh
• rebuilt the VM image and verified wait_for_ib_gateway_ready.sh paper returns ready
• manually reran interactive-brokers-quant-service-scheduler and confirmed execution report 20260422T064704Z.json finished with status ok