Replace the WebSocket-over-TCP IPC between vscode and R sessions with
a Unix domain socket (Linux/Mac) or Windows named pipe transport.

Changes:
- session.ts: create net.Server pipe instead of WebSocket.Server;
  use NDJSON framing (\n-delimited JSON) for message boundaries;
  remove token-based authentication (filesystem permissions suffice)
- rTerminal.ts: pass SESS_PIPE env var instead of SESS_PORT + SESS_TOKEN
- sess/R/server.R: connect via processx::conn_connect_unix_socket();
  drive message dispatch through a later::later() polling loop
- sess/R/dispatch.R: send via processx::conn_write() + NDJSON framing
- sess/DESCRIPTION: swap websocket dep for processx (>= 3.5.0);
  add testthat to Suggests
- sess/tests/testthat/test-ipc.R: unit tests for socket round-trip,
  dispatch_message routing, and ipc_write guard