Cross-instance message/event switching plugin

Key features

• send / ping HTTP endpoint (/interconnect/): deliver messages across instances and machines, detect activity
• Both plug-ins are hung on host composition: interconnect is a cross-session, cross-machine process-level service (with HTTP/WS endpoints) and must be host-level
• 22/22 Single test passed (service 17 + tool 5)
• /interconnect/link WebSocket endpoint: two-way real-time event streaming, including heartbeat and exponential backoff reconnection
• Event fan-out (HTTP + WebSocket), inbound events emitted as interconnect/event