Currently, the read_gen field of a page (WT_PAGE) is owned by the eviction module but is accessible and modifiable by modules outside of eviction. To improve encapsulation we should make read_gen private to the eviction module. This means any module outside of eviction should interact with read_gen solely through designated APIs, without direct modification.

This change will:

• Ensure that only the eviction module can modify read_gen directly.
• Encourage the use of established (or created new) APIs for modifying page state.

This task is an example of the work described in WT-13604 which is scheduled for post Q3.