Add unit action: annex or puppet a City-State

[Fanfblrik]

Summary

• Engine support for RekMOD Merchant of Venice (Great Merchant UU that can buy a City-State by consuming the unit).
• New unit action unique: Can annex or puppet a City-State (typically with <by consuming this unit>).
• Allowed when the CS is unallied or already allied to the buyer, not at war, and not on Diplomatic Marriage cooldown.
• Shares takeover logic with Austria-style Diplomatic Marriage; Austria's Gold + Ally-turns unique is unchanged and separate.

Test plan

• Unit with the unique on a CS tile can use Buy City-State when the CS is unallied or your ally.
• Action unavailable vs enemy Ally CS / at war / marriage cooldown.
• Consuming the unit annexes/puppets as with Diplomatic Marriage alerts for humans.

[yairm210]

Why?

[Fanfblrik]

This is a unit action (Merchant of Venice on the CS tile, typically <by consuming this unit>), not a generalization of Austria's civ-wide Gold marriage. Austria stays: spend Gold after N Ally turns + cooldown. Venice: consume the merchant, no Gold / no Ally-turn gate, allowed when the CS is unallied or already your Ally; takeover path is shared with Diplomatic Marriage. Happy to rename/clarify the unique text if the wording is the concern.

[yairm210]

Still unclear why it's required

[Fanfblrik]

Needed for RekMOD (LekMOD-style) Merchant of Venice: a Great Merchant UU that can buy/annex a City-State by consuming the unit on its tile. That is not expressible with Austria’s Gold + Ally-turns unique, so this UnitAction unique is the engine piece for that nation ability.