opt(admin): SO_REUSEPORT per-worker listeners for multi-worker startup

[zhongwen666]

closes #1266

Summary

Give each Uvicorn worker its own SO_REUSEPORT listener when the admin service runs with multiple workers, so the kernel load-balances connections across workers instead of sharing one parent-bound socket.

Key changes

• Add _create_reuse_port_socket / _ReusePortServer / _run_reuse_port_workers in rock/admin/main.py: each worker binds its own listener into a SO_REUSEPORT group after fork.
• Route main() to the new path when workers > 1; keep uvicorn.run for single-worker startup.
• Parent preflights the bind and fails fast on unsupported setups (non-Linux, UDS/fd listener, or port 0) with clear logging.