chanserv: SET TOPICLOCK and KEEPTOPIC (topic subsystem)

Adds topic tracking: a TopicChange event (parsed from InspIRCd FTOPIC, our own
changes filtered by SID like FMODE) and a stored topic on the channel, folded
through the event log. TOPICLOCK reverts a topic change by a user without
op-level access back to the stored topic; KEEPTOPIC remembers the topic and
restores it when a registered channel is recreated. Both via the typed
ChanSettings path; shown in INFO. Parse + engine tests cover both.

src/grpc.rs

@@ -130,7 +130,8 @@ fn to_wire(entry: &LogEntry) -> Option<ReplicationEvent> {
         | Event::ChannelAkickAdd { .. }
         | Event::ChannelAkickDel { .. }
         | Event::ChannelEntryMsgSet { .. }
-        | Event::ChannelSettingsSet { .. } => return None,
+        | Event::ChannelSettingsSet { .. }
+        | Event::ChannelTopicSet { .. } => return None,
     };
     Some(ReplicationEvent { origin: entry.origin().to_string(), seq: entry.seq(), lamport: entry.lamport(), kind: Some(kind) })
 }