Adds test_rcfile_integration.sh, mirroring clean_failover_preservation.sh's
shell-driven black-box pattern against the real built xce_test binary:
- a valid rc file's set masters override takes effect in both a piped run
and a genuine pty-driven interactive run, before the normal session begins
- --norc leaves the default masters value untouched with the same rc file
present
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg
