statserv: persist per-channel activity across restart too

The earlier StatsSet fix persisted only the shared namespaced counters. The
per-channel BOTSTATS view (lines seen + top talkers, from /statserv #chan) lived
on the LiveChannel struct, which is rebuilt from each burst — so it reset to zero
on restart. Move it to a name-keyed Network.chan_activity map (independent of live
membership, seedable before the burst) and carry it in the StatsSet snapshot
(new serde-default `channels` field) alongside the counters. Reworded the
notice from 'seen this session' now that it persists.

src/engine/db/network.rs

@@ -250,10 +250,24 @@ impl Db {
         self.net.stats.clone()
     }
 
-    /// Snapshot the live stat counters to the log so they survive a restart.
-    pub fn persist_stats(&mut self, counters: &std::collections::BTreeMap<String, u64>) -> std::io::Result<()> {
-        self.log.append(Event::StatsSet { counters: counters.iter().map(|(k, v)| (k.clone(), *v)).collect() })?;
+    /// The persisted per-channel activity, to seed BOTSTATS on startup.
+    pub fn persisted_chan_stats(&self) -> ChanStats {
+        self.net.chan_stats.clone()
+    }
+
+    /// Snapshot the live stats (shared counters + per-channel activity) to the log
+    /// so they survive a restart.
+    pub fn persist_stats(
+        &mut self,
+        counters: &std::collections::BTreeMap<String, u64>,
+        chan_stats: ChanStats,
+    ) -> std::io::Result<()> {
+        self.log.append(Event::StatsSet {
+            counters: counters.iter().map(|(k, v)| (k.clone(), *v)).collect(),
+            channels: chan_stats.clone(),
+        })?;
         self.net.stats = counters.clone();
+        self.net.chan_stats = chan_stats;
         Ok(())
     }