feat(core): report the bound port over IPC

[harijoe]

Summary

• server.run() reports its bound port to the parent over IPC as { type: "skybridge:listening", port }.
• Only fires when the process was spawned with an IPC channel, so every production path is unaffected.

Architecture Notes

• Why IPC rather than a return value — binding and reporting both happen in the child, so a caller can use __PORT=0 and learn the real port without a window where another process can take it.