Disallow event creation if the self node is not in the current roster

This is related to a TSS upgrade that inculdes an address book change. For nodes that are not part of the new address book after a freeze:

1. These nodes do not create any new events, because they are not in the current address book, and they will all collect enough signatures to fully sign the freeze block from the events created prior to the upgrade with the BR 100 events.
2. These nodes will continue gossiping, but not creating new events, until the freeze round is ancient.

Start by locating the event-creation path and the logic that determines the self node's current roster or address-book membership. Trace how freeze rounds and gossip are handled, then identify the existing tests for event creation; done means an out-of-roster node creates no new events while continuing to gossip until the freeze round is ancient.

Written by the indexing model from the issue text.