Three things stood in the way of two people sharing a cab and of a client not
being able to reach past its own vehicle.
Vehicle commands were addressed by TTrain::id(), which every peer hands out
locally from its own counter, so the same number means a different vehicle on
a different peer. Commands now travel with the session wide NetworkEntityId
and are translated back to the local cab on arrival; outside of multiplayer
the registry is empty and both directions are a no-op, so singleplayer is
untouched.
A client could send any command it liked, including setweather, queueevent or
spawntrainset. The server now validates every incoming command: a player may
work the controls of the vehicle whose crew they are on and nothing else, and
world wide commands stay with the host until dispatcher roles exist. The
source is stamped by the server, never taken from what the peer sent, and
rejections are logged once per peer/command instead of every frame. Host input
runs through the very same validation, just without the transport.
Continuous commands were tracked per command and recipient only, so with two
people in one cab the release sent by one of them cancelled the other one's
hold. command_data now carries the source peer and the active set is keyed by
{source, recipient, command}.
Also: a client asking to quit no longer replicates quitsimulation, which would
have shut down everybody else's session.
The zmq interface kept its own copy of the state field list, its own name
table and its own packer. All three are now the shared registry, which
resolves the same channel names and returns the same values, so the wire
format is unchanged. New state exposed to external interfaces is added in
one place from now on.
The registry reports the raw velocity rather than zeroing it while the
simulation is paused, and a symbol which doesn't apply to the current
vehicle keeps its measurement and says so through its quality flag.
Consolidates platform-specific config-directory logic (APPDATA on Windows,
~/Library/Application Support on macOS, ~/.config on Linux) into a single
user_config_path(filename) function in utilities/utilities.{cpp,h}, removing
copy-pasted #ifdef blocks from application.cpp, editor/editorSettings.cpp,
input/{keyboard,gamepad,drivermouse,uart}input.cpp.
