Merge PR #471 from NikitkaRa: Control server: bound connections, line length, queue and socket timeouts (#418)

docs/control-protocol.md

@@ -3,6 +3,11 @@
 Start the app with `--control <port>`. The server listens on `127.0.0.1` only and speaks JSON
 lines: one request object per line, one reply per line.
 
+The server bounds what a client can hold: 16 connections at once (more get `{"ok":false,"error":"busy: …"}`
+and are closed), 16 MiB per request line (longer closes the connection), 64 queued requests (more are
+answered `busy`), 10 minutes idle and 30 seconds per blocked reply write before the socket is closed, and
+60 seconds waiting for the UI (`"error":"timeout"`).
+
 ```json
 {"id": 1, "method": "engine.execute", "params": {"command": "shape.insert", "params": {"preset": "roundRect", "rect": [100, 100, 240, 120], "text": "Hello"}}}
 {"id": 1, "ok": true, "result": {"id": 258}}