Main now snapshots systemd unit listings before stopping old processes
and passes them into _restart_systemd_gateway_units_best_effort; the
catch-up budget test and eval call the new two-argument shape while
still asserting the unit's stop+start budget reaches the client timeout.
Salvage the unit-budget implementation from #104745, replacing its test
matrix with two invariant tests and covering the sibling graceful start.
Keep unprivileged property reads, finite fallbacks, real manager errors,
and post-restart health verification.
Native disposable user unit: old client timed out after 15.03 seconds;
new client completed the same 16-second stop transaction in 16.13 seconds.
The unit stayed active with a new PID; missing-unit errors stayed errors.
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>
Teknium