Release v2.1.0: backend test seam, captive-portal detection, JSON status, robustness

Features:
- Introduce a Backend trait + System impl so flow/status/watch can be unit
  tested against a fake; add 11 connect-state-machine tests.
- Captive-portal detection: status::connectivity returns Online/Portal/Offline;
  surfaced in status, JSON, connect notes, and a dedicated watch state.
- `status --json` for bars/scripts; `profile add`/`profile remove`; detect now
  scores by number of in-range markers.

Robustness:
- Pin LC_ALL=C/LANG=C on child processes for locale-independent parsing.
- Atomic config/state writes (temp + rename); 0600 config never world-readable.
- Transient PSK file written to $XDG_RUNTIME_DIR when available.

Fixes (from prior audit):
- Feed Wi-Fi PSK to nmcli via stdin/passwd-file, never argv.
- mask() no longer panics on multi-byte passwords.
- Connectivity check requires HTTP 204 (no captive-portal false positives).
- nmcli NAME,TYPE parsing handles escaped colons.
- Strip CIDR suffix from displayed IP; PKGBUILD/Cargo version aligned (2.1.0).

breadcrumbs.example.toml

@@ -11,6 +11,8 @@ nmcli_wait = 8
 exit_node = "my-exit-node"     # Tailscale hostname of your preferred exit node
 default_profile = "away"
 watch_interval = 12
+# Must be a "generate_204"-style endpoint: only an empty HTTP 204 counts as
+# online, so a captive portal (200 login page / 30x redirect) is detected.
 connectivity_url = "http://connectivitycheck.gstatic.com/generate_204"
 ping_host = "1.1.1.1"