Demote 35 info-level log messages to debug for cleaner production output

Reduce info-level noise by moving intermediate steps, periodic
telemetry, cross-connection resolution details, and redundant messages
to debug. Info output now focuses on operator-relevant state changes:
lifecycle events, peer promotions, session establishment, parent
switches, and transport start/stop.

Key categories demoted:
- Handshake cross-connection resolution mechanics (10 messages)
- Periodic MMP link/session metric reports (4 messages)
- TUN cleanup messages redundant with lifecycle shutdown (4 messages)
- Transport "packet channel closed" shutdown messages (4 messages)
- Retry scheduling, discovery lookup initiation, other intermediate steps

Change default RUST_LOG from debug to info in systemd unit files.

src/node/tree.rs

@@ -198,7 +198,7 @@ impl Node {
 
         self.stats_mut().tree.accepted += 1;
 
-        info!(
+        debug!(
             from = %self.peer_display_name(from),
             seq = announce.declaration.sequence(),
             depth = announce.ancestry.depth(),