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.
