fix(server): preserve routed outbound messages - #231

Summary

• replace lossy fan-out queues with FIFO outbound mailboxes and exclusive receiver leases
• preserve unread messages across SSE receiver handoff and return 409 for overlaps
• drain naturally completed connections while keeping explicit shutdown abortive
• route HTTP by connection or session and WebSocket through one aggregate mailbox
• require session headers for every existing-session method, including NES and document notifications

HTTP JSON-RPC batches remain intentionally unsupported.

Testing

• npm run check (862 tests)