Second cross-fleet shared-library adoption (after Health). Full scope:
AddZbTelemetry (OTel Resource identity triple + standard instrumentation +
Prometheus /metrics) on all 3, plus shared Serilog on all 3 — including the
MxGateway MEL->Serilog migration. Records the correction that MxGateway's
logging was NOT actually adopted on main despite the docs' claim. Behaviour-
preserving bar; breaking items (#6 unit, #7 rename) deferred.
Joseph Doherty
3729ff2152