Coastal Immigration Beacon v0.2.7

The vanilla Beacon now requires a valid coastal ocean berth. Its original
workers, power, Unity, progress, rewards, and population logic remain vanilla.
An existing inland Beacon remains in the save but immigration is stopped until
the Beacon is rebuilt correctly on the coast.

The immigrant fishing boat is visual and transient. It is removed before every
save and never participates in fleet limits or save serialization.

Fishing boat model by crookedmouth, distributed under CC0:
https://opengameart.org/content/fishing-boat-low-poly
