chore(Mountain/Vine): Add IPC module placeholder

- Introduces empty `Ipc` module structure in Mountain backend to establish foundation for Vine IPC implementation
- Aligns with MVP Path A requirements for Cocoon sidecar communication via newline-delimited JSON over stdio
- Prepares for future implementation of Mountain-side IPC handlers to route extension requests through Track dispatcher
- Contributes to core architecture principle of maintaining process isolation between Mountain and extension hosts
- Empty module initialization follows Rust project conventions while marking integration point for Vine transport layer

Author: NikolaRHristov