botserv: op assigned bots on join; statserv: persist counters across restart

Problem 1: an assigned bot joined via IJOIN with no status mode, so it had no
+o. Join it with the 4-param IJOIN form (ts 1, mode o) so it's opped, matching
Anope's default bot modes (+o), and exempt bots from SECUREOPS/RESTRICTED so the
op is never stripped.

Problem 2: the shared stat counters lived only in memory, so a restart wiped
them. Snapshot them to the log (StatsSet event, folded into NetData + compaction)
periodically and on shutdown, and reseed the live registry from the log at
startup. Live gauges (online counts) stay re-derived, not stored.

src/engine/state.rs

@@ -349,6 +349,11 @@ impl Network {
         &self.stats
     }
 
+    // Seed the counters from persisted state at startup, so they survive restarts.
+    pub fn seed_stats(&mut self, stats: BTreeMap<String, u64>) {
+        self.stats = stats;
+    }
+
     // BOTSTATS view: (total lines, top talkers by count, descending).
     pub fn channel_activity(&self, channel: &str) -> Option<(u64, Vec<(String, u64)>)> {
         let c = self.channels.get(&lc(channel))?;