JNR's Khmer Tweak (Permanent Yield Preservation)

I didn't like how the Khmer's bonus that preserved yields on floodplains with districts was lost in later ages, making it somewhat meaningless to lean into it. This mod moves that bonus to the Baray, making it ageless, while restricting it to cities the Khmer built. In return, the Baray's +1 Food to Floodplains in the city is now the Khmer's base ability instead.