Review frequency of city/port connectivity check with factory

Revealed in https://github.com/openfrontio/OpenFrontIO/pull/5246#issuecomment-5538844226

It is found that Cities and Ports check for factory connectivity at different frequencies. Specifically, Cities only check once at creation, but Ports check every tick.

Is this intended? Should both units check connectivity at every tick? If performance is a concern, then when it is possible for players to spam Ports (e.g. coastal area), should Ports just check connectivity only once when placed?

Start with the city and port factory-connectivity checks described in PR 5246's issue comment, then trace when each check runs during creation and during ticks. Compare the behavior in coastal placement and repeated-port scenarios, and determine the intended frequency before documenting or implementing a consistent rule.

Written by the indexing model from the issue text.