Make sure featureVisibility modrule applies only at spawn

Apparently features made visible via this modrule are just visible, period (i.e. `Spring.SetFeatureAlwaysVisible(featureID, false)` doesn't make them obey fog of war). Check whether this is the case and make the modrule just set the alwaysVisible state once at spawn.

Start by tracing the featureVisibility modrule and the feature spawn handling in the engine. Verify whether Spring.SetFeatureAlwaysVisible(featureID, false) restores fog-of-war behavior, then confirm that the modrule sets the always-visible state only once at spawn and that later changes obey fog of war.

Written by the indexing model from the issue text.