Add upon gifting unit to City-State trigger

[Fanfblrik]

Summary

• Engine support for RekMOD Sweden (Nobel Prize: Gold when gifting a Great Person to a City-State).
• New trigger: upon gifting a [mapUnitFilter] unit to a City-State.
• Fired from the City-State gift action (before destroy/gift).
• Compose as Gain [200] [Gold] <upon gifting a [Great Person] unit to a City-State>.
• Non-military units matching such a trigger are giftable to City-States (same allowance path as the Influence gift unique).
• Existing Influence gift unique left as-is.

Test plan

• Civ with Gold gift trigger: gift GP to CS → +200 Gold; Influence gift unique still applies if present.
• Without matching filter: no Gold from this trigger.
• Military gift path unchanged.

[yairm210]

Why?

[Fanfblrik]

Good catch — a dedicated gift-stat UniqueType was the wrong shape. Replaced with trigger upon gifting a [mapUnitFilter] unit to a City-State, so Sweden is Gain [200] [Gold] <upon gifting a [Great Person] unit to a City-State>. Existing Influence gift unique left as-is for now.

[Fanfblrik]

Clarifying motivation: this is for RekMOD Sweden (Nobel Prize — Gold on Great Person gift to a City-State). The trigger shape lets that nation UA compose on existing Gain [stat] without a one-off gift UniqueType.