Migration mechanic

Expand on the migration mechanic by having "migration pressure" be factored into population growth.

Cities nearing their housing limit would generate negative pressure, which would reduce population growth in the nearly full city but proportionally boost growth in underpopulated territories nearby.

Centers connected through trade route would be considered "closer" for this purpose, meaning a trade hub will see a lot of competiting pressures.

These pressure would be slow, so sudden changes to food production would still cause famine (and refugees migrating if you have the diplomacy agreement with someone else) but would otherwise limit the amount of warnings players get when their cities reached their limit.

This would also allow for interaction with other mechanics; open-borders, territory's dominant faith & culture, trade route, siege (low stability city will have more refugee fleeing, reducing the number of militias left available to defend it).

Agrarian migration may be kept as is or need reworking as it might become difficult to hold on to the attracted migrants.

A city gaining population through migration more than fertility may have various effect happen (modifier to certain yield, like decreased influence & faith due to having to teach the newcomers about your customs and laws but other yields are increased slightly, maybe pressure to become independent/switch allegence if the target empire has a relevant grievance).