Promote BotServ BOTSTATS to a StatServ pseudo-service

Stats now have their own service. StatServ answers /msg StatServ <#channel>
(per-channel activity — lines + top talkers, founder-or-admin) and
/msg StatServ SERVER (the shared cross-service counter registry + gauges,
operators only). The shared counter map moved from the engine onto the
network view so both StatServ (via NetView::stat_counters) and the gRPC
Stats API read one source. BotServ's BOTSTATS command is removed; the
per-channel activity is still recorded in the dispatch loop.

statserv/src/lib.rs

@@ -0,0 +1,56 @@
+//! StatServ surfaces statistics: the shared, cross-service counter registry
+//! (SERVER, operators only) and per-channel activity the bots have seen
+//! (a #channel argument, for its founder). `lib.rs` holds the dispatcher; each
+//! view lives in its own file.
+
+use fedserv_api::{NetView, Priv, Sender, Service, ServiceCtx, Store};
+
+#[path = "global.rs"]
+mod global;
+#[path = "channel.rs"]
+mod channel;
+
+pub struct StatServ {
+    pub uid: String,
+}
+
+impl Service for StatServ {
+    fn nick(&self) -> &str {
+        "StatServ"
+    }
+    fn uid(&self) -> &str {
+        &self.uid
+    }
+    fn gecos(&self) -> &str {
+        "Statistics Service"
+    }
+
+    fn on_command(&mut self, from: &Sender, args: &[&str], ctx: &mut ServiceCtx, net: &dyn NetView, db: &mut dyn Store) {
+        let me = self.uid.as_str();
+        match args.first().copied() {
+            Some(chan) if chan.starts_with('#') => channel::handle(me, from, chan, ctx, net, db),
+            Some(cmd) if cmd.eq_ignore_ascii_case("SERVER") || cmd.eq_ignore_ascii_case("GLOBAL") => global::handle(me, from, ctx, net, db),
+            Some(cmd) if cmd.eq_ignore_ascii_case("HELP") => help(me, from, ctx),
+            None => help(me, from, ctx),
+            Some(other) => ctx.notice(me, from.uid, format!("I don't know \x02{other}\x02. Try \x02SERVER\x02, a \x02#channel\x02, or \x02HELP\x02.")),
+        }
+    }
+}
+
+fn help(me: &str, from: &Sender, ctx: &mut ServiceCtx) {
+    ctx.notice(me, from.uid, "StatServ reports statistics: \x02SERVER\x02 for network-wide counters (operators), or \x02<#channel>\x02 for a channel's activity (its founder).");
+}
+
+// Shared gate: the sender may see a channel's stats only as its founder or a
+// services admin.
+fn require_channel_admin(me: &str, from: &Sender, chan: &str, ctx: &mut ServiceCtx, db: &dyn Store) -> bool {
+    let Some(founder) = db.channel(chan).map(|c| c.founder) else {
+        ctx.notice(me, from.uid, format!("\x02{chan}\x02 isn't registered."));
+        return false;
+    };
+    if from.account != Some(founder.as_str()) && !from.privs.has(Priv::Admin) {
+        ctx.notice(me, from.uid, format!("Only \x02{chan}\x02's founder can see its stats."));
+        return false;
+    }
+    true
+}