2026-July-12 - 2026-07.16.15682

Make TechnologyFrontier#addAdvance idempotent (#14718)

* Fix IllegalStateException in TechnologyFrontier#removeAdvance

Make removeAdvance idempotent instead of throwing when the advance
is already absent. This can legitimately happen when multiple map
triggers attempt to remove the same tech in the same event, as seen
in the invasion_usa map (issue #14635).

* Make TechnologyFrontier#addAdvance idempotent

Follow-up to #14684. Applies the same idempotent pattern to addAdvance
that was used for removeAdvance, so that adding a TechAdvance that's
already present is a safe no-op instead of creating duplicate entries
in the techs list.

---------

Co-authored-by: frigoref <frigoref@web.de>