[alandarev]

Summary

Adds console (text) to the developer control socket: it runs one $ command
as if typed into the chat box, without opening the chat box, so scripted
screenshots and checks can switch a developer option ($fpscounter on, …) and
nothing typed shows up in the frame. A text that is not a $ command the client
knows, or one whose arguments are missing or malformed, answers bad_request
(a $ handler that throws on a bad argument, such as $fps without a number,
no longer ends the client when it comes through the socket). Documented in
docs/control-socket.md.

Related issues

Follow-up to the control socket (#594).

Testing

Rebased onto main 112cece (after #610). main does not build on Linux
(#730), so the Linux runs below add #719's build fixes (fork branch
fix/linux-gcc-build 9ecbf65) under this branch, test-only.

• cmake --build --preset linux-x64-release + ctest: 603/603.
• clang-format 21.1.8 and Cppcheck with the CI options: no findings.
• CI on the fork for this head, merged with main 2e41b7c: Quality Gates and the
  Windows editor-OFF build with its tests pass (https://github.com/alandarev/mumain-sven/actions/runs/38795383439,
  https://github.com/alandarev/mumain-sven/actions/runs/38795383352). The other jobs fail on main too, with the same errors
  (Build fails on main (112cece4d): macOS, Windows/Linux with the editor, Linux; CI has not run on main since August #730).
• Live: console '$fpscounter on' and console '$fps 60' → ok;
  console '$fps' (no number) → bad_request, and the client keeps running (it
  used to end the client, through the socket as through the chat box);
  console '$nosuchcommand' → bad_request.

Written with AI assistance and tested by hand.

Checklist

• I have read and followed docs/CODING_RULES.md.
• I have built the project locally (see docs/build/README.md).
• Changes are focused on one concern; the diff is as small as it reasonably can be.