feat: diagnostic log (tracing)

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.

rust/core/Cargo.toml

@@ -23,6 +23,7 @@ hmac = "0.13.0"
 sha1 = "0.11.0"
 zeroize = "1"
 parking_lot = "0.12"
+tracing = "0.1"
 portable-pty = "0.9.0"
 serialport = { version = "4", default-features = false }
 des = "0.9"