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/handlers/timeout.rs

@@ -33,13 +33,13 @@ impl Node {
                 let direction = conn.direction();
                 let idle_ms = conn.idle_time(now_ms);
                 if conn.is_failed() {
-                    info!(
+                    debug!(
                         link_id = %link_id,
                         direction = %direction,
                         "Failed handshake connection cleaned up"
                     );
                 } else {
-                    info!(
+                    debug!(
                         link_id = %link_id,
                         direction = %direction,
                         idle_secs = idle_ms / 1000,