feat: connectivity state and captive-portal URL surface

[cachebag]

This PR xposes NM's Connectivity property and CheckConnectivity() method through a typed API.

• ConnectivityState enum mapping NM's NMConnectivityState (Unknown/None/Portal/Limited/Full)
• ConnectivityReport aggregating state, check-enabled flag, check URI, and captive-portal URL
• connectivity() for fast single-property reads, check_connectivity() to force a probe
• captive_portal_url() with best-effort detection via IP4Config properties (newer NM) falling back to the configured check URI
• ConnectivityCheckDisabled error variant when checks are off