v0.4.3

A build-hook fix so the package installs on hosts that lack the native-bridge
toolchain.

• Skip the native bridge cleanly when its build prerequisites are absent.
  The bridge needs a C++ toolchain (cmake + ninja) and libsystemd's sd-bus;
  on Alpine/OpenRC, a minimal image, or any non-systemd host, the build hook
  now probes for these up front and returns without building — the PackageKit
  backend is simply unavailable, which is fine where it is unused. Previously a
  missing tool threw ProcessException (only a present-but-failing tool was
  handled), failing the whole build hook and blocking dart pub get / install
  for every consumer on such a host. Probing first also avoids a needless
  sdbus-cpp clone.