feat!: saved connection enumeration (Settings / Connection proxies)

[cachebag]

This PR adds typed enumeration of NetworkManager saved profiles via org.freedesktop.NetworkManager.Settings and Settings.Connection, with decoded SettingsSummary (Wi‑Fi, Ethernet, VPN, WireGuard, GSM, CDMA, Bluetooth, Other).

API

• SavedConnection, SavedConnectionBrief, SettingsPatch, SettingsSummary, WifiSecuritySummary, WifiKeyMgmt, VpnSecretFlags
• list_saved_connections, list_saved_connections_brief, list_saved_connection_ids, get_saved_connection, get_saved_connection_raw, delete_saved_connection, update_saved_connection, reload_saved_connections
• Errors: SavedConnectionNotFound, MalformedSavedConnection
• Proxies: NMSettingsProxy, NMSettingsConnectionProxy
• Example: saved_list
• list_saved_connections now returns Vec<SavedConnection> (was Vec<String>). Replacement for name-only list: list_saved_connection_ids.

Internal

• Removed list_saved_connections from connection_settings (superseded by core/saved_connection).
• List path uses buffer_unordered(16) over ListConnections + per-path GetSettings + decode; malformed profiles are skipped with warn!.