The app now writes a diagnostic log to nexus.log in the data directory (0600, rotated at 5 MiB): connection attempts and results, remote closes, reconnect scheduling, host-key/certificate checks with the policy applied, and credentials that couldn't be stored (kind only). Credentials and terminal data are never logged. RUST_LOG sets the level and also mirrors the log to stderr. The remaining eprintln! diagnostics in storage and URL handling go through it too.