Front the whole network's help through HelpServ

Each service now exposes its help catalog via Service::help_topics; the
engine collects them into a network-wide index on Network, reachable
through two new NetView methods. HelpServ uses it: HELP <service>
[command] (or a bare service name) serves any service's per-command
help, bare HELP lists the services, and its own ticket commands still
work. No cross-crate dependency: the catalog flows through the SDK's
NetView, keeping every module dependent only on echo-api.

modules/chanfix/src/lib.rs

@@ -43,6 +43,10 @@ impl Service for ChanFix {
         "Channel Fixer"
     }
 
+    fn help_topics(&self) -> (&'static str, &'static [HelpEntry]) {
+        (BLURB, TOPICS)
+    }
+
     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();
         // The whole service is operator-only.