variants/linux: control-socket console and meshcorectl

[mmmorks]

Staged on the fork for review. Final destination: l5yth/meshcore-linux linux (competes with upstream l5yth#25; may be reworked). Base here is pr/04-event-loop-cad so the diff shows only this change.

Summary

On Linux the Arduino Serial object is an output-only stub, so the firmware's serial CLI printed but could never receive a command: a packaged meshcored was configurable only over the mesh or through meshcored.ini. This adds LinuxConsole, a Stream the CLI reads from and writes to, backed by a Unix-domain control socket (or stdin when run in a foreground terminal), and meshcorectl, a dependency-free Python client for it.

Relationship to l5yth#25. @rgrizzell's l5yth#25 solves the same problem with a pseudo-terminal, and the two are alternative designs rather than complementary ones — both are Linux-only, both hook the same point in main.cpp. The trade-offs, as neutrally as I can put them:

• variants/linux: local CLI console over a PTY l5yth/meshcore-linux#25's PTY is usable by existing terminal tools with no client of its own: meshcore-cli -r -s, minicom, screen. That matters on hosts too small for Python.
• This one is a socket with a client. That buys a REPL with line editing, history and Tab completion of the real command set, one-shot and piped modes with meaningful exit codes (0 only if every command was actually run by the daemon — including reboot, whose only reply is its echo), a single-client rule with an explicit ERR: control socket busy refusal instead of silently queueing a second client's command, a liveness probe so a second daemon cannot steal a live socket, close-on-exec descriptors so a reboot re-exec does not inherit the listener, and the socket path resolution (RuntimeDirectory → $XDG_RUNTIME_DIR → /tmp) with the permissions story written down. Raw tools still work (socat - UNIX-CONNECT:...).

Happy for the maintainer to pick one. If l5yth#25 is preferred, the parts of this PR worth keeping regardless are PeekableStream (also used by the GPS stream PR) and the descriptor registration with the event loop; if this one is preferred, exposing a PTY alongside the socket for meshcore-cli is a contained addition to LinuxConsole.

What changed

• variants/linux/LinuxConsole.{h,cpp}: the console. A live socket client takes priority over stdin. The socket is created at mode 0600 under umask(0177) and widened to 0660 afterwards (no window at umask-derived permissions). try_bind() probes each candidate path with connect() first and only reclaims one that answers ECONNREFUSED; it refuses to unlink anything that is not a socket (the path is operator input via MESHCORED_CONTROL_SOCKET). A client in a persistent read-error state is dropped rather than retried at 1 kHz; a second concurrent client is answered and closed. registerPollFds() tells the event loop exactly which descriptors rawReadByte() will drain next, so the daemon wakes on console traffic and on nothing else.
• variants/linux/PeekableStream.h: the one-byte-lookahead Stream idiom (available()/peek()/read() over a non-blocking descriptor), so subclasses supply only rawReadByte().
• variants/linux/meshcorectl: the client. Completion tables checked against the dispatch literals in CommonCLI.cpp and MyMesh.cpp (get and set have separate key pools; region subcommands listed). Its docstring records the firmware behaviours it cannot fix (clock sync needs a sender timestamp the socket path passes as 0; region load is multi-line).
• examples/simple_repeater/main.cpp: the CLI reads from Console on Linux (MC_CLI, Serial elsewhere); Console.begin() in setup().
• variants/linux/LinuxBoard.{h,cpp}: reboot() tears the console down before the re-exec; idleUntilEvent() registers the console's descriptors.
• variants/linux/meshcored.service: RuntimeDirectory=meshcored (mode 0750), so /run/meshcored/meshcored.sock exists and is reachable from outside PrivateTmp.
• test/test_linux_console: lifecycle over real sockets in a temp dir — accept and echo, hangup releasing the listener, persistent read error, the busy refusal, the non-socket refusal, both path-length reports, close-on-exec, and PeekableStream's contract. LinuxConsole.cpp joins the native build, which is why MSG_NOSIGNAL is guarded (macOS uses SO_NOSIGPIPE).
• README: meshcorectl installed alongside meshcored; a ## The control CLI section (socket path order, permissions, the /tmp fallback's caveat, the three ways to drive the client, the one-client rule, stdin when run in a terminal).

How it was tested

• Native suite passes with the new console tests; each fix in the hardening rounds was confirmed to fail its test when reverted.
• Clean linux_repeater build for arm64 in the container; two daemons run concurrently in the container to confirm the first keeps its socket (same inode) and the second declines it, and that a stale socket is reclaimed once its owner dies.
• meshcorectl exercised against a stand-in socket for each failure mode (live, busy, silent, dying mid-script, missing) and end to end against the real console in one-shot, piped and REPL form on a Pi.

Dependencies

Stacked on pr/04-event-loop-cad (the console registers its descriptors with the event loop, and reboot()'s teardown assumes that PR's LinuxBoard.cpp). Review the last commit only until that merges. pr/06-gps adds an identical PeekableStream.h; git merges identical additions cleanly.

Shared code touched

• examples/simple_repeater/main.cpp (a #define selecting Console on Linux and Serial everywhere else; no change for other targets)
• platformio.ini (native test env only)

[mmmorks]

Superseded by #10, which keeps upstream l5yth#25's PTY console and layers this PR's hardening, stdin input and meshcorectl on top of it. Branch pr/05-control-socket-console stays for reference.