Vala GLib.DateTime local wall-clock parsing during DST overlap

Goal: diagnose which instant Vala selects when converting an offset-less local wall-clock timestamp to UTC through GLib.DateTime during a DST fall-back overlap, where one local time can map to two UTC instants.

Constraint: Vala relies on the host system's tzdata via GLib.TimeZone and does not expose an explicit disambiguation parameter in the standard DateTime API. Interpretation may depend on the GLib minor release and whether the deployment ships current zoneinfo, especially in containers.

Version assumption: GLib 2.7x or later with Vala bindings for timezone-aware DateTime.

• Does GLib consistently favor the earlier or later offset during a fall-back overlap, and is that behavior documented as stable?
• How can a Vala application determine which offset was chosen after parsing an offset-less local timestamp?
• What is the safest pattern for regional conversion when tzdata may be outdated or absent?