So that WT-16512 remains possible, we should reserve the unused field in the page header to support future extensibility.

We should do the following:

• Rename the unused field to reserved.
• Add a comment with FIXME-WT-16512 for that field that explains its future intended use case.

This would prevent someone else from reusing the field for a different purpose, which would prevent us from making the header extensible in the near future.