Merge origin/main into issue-525

docs/control-protocol.md

@@ -15,6 +15,10 @@ Then send newline-delimited JSON to `127.0.0.1:7979`. Every request line gets on
 
 Errors come back as `{"id": …, "ok": false, "error": "…"}`.
 
+A line that is not a JSON object with a string `method` (an HTTP request, text that isn't JSON,
+bytes that aren't UTF-8, a line over 64 MiB) gets one error reply and the server closes the
+connection; nothing sent after it runs. Blank lines are skipped.
+
 ## Methods
 
 | Method | Params | What it does |
@@ -65,3 +69,18 @@ the password, and never replaces the encrypted original.
 
 For a headless equivalent (no window), use `gridcraft-cli mcp` or `gridcraft-cli run`;
 see [`mcp.md`](mcp.md) and [`cli.md`](cli.md).
+
+### Native printing
+
+Desktop `engine.commands {"search":"print"}` (and connected MCP `list_commands`)
+lists `ui.printSettings` and `ui.printSubmit` with their parameter and result fields.
+
+`engine.execute` can call `ui.printSettings` with `{}` to inspect the printer list, selected
+printer, copies, duplex, grayscale, discovery/submission status and backend receipt/error.
+`{"refresh":true}` starts background discovery; poll the same command until `discovering` is
+false. Set `printer` to a discovered name, integer `copies` from 1 to 999, `duplex` to
+`printerDefault`, `off`, `longEdge` or `shortEdge`, and boolean `grayscale`. Invalid updates are
+rejected without changing earlier settings. `ui.printSubmit` explicitly submits the current
+print-ready PDF without opening a dialog; inspect `ui.printSettings` until `busy` becomes false
+and check `status`/`detail`. The browser keeps its own print hook. PDF export remains separate.
+GridCraft accepts the existing `file.print` sheet/pagination params on `ui.printSubmit`; the engine `file.print` command returns the PDF without spooling.