sessiond: never reapply clat — it destroys the carrier's own routes

Measured on the phone. After one PreferLink the clat device was up with its
192.0.0.1/32 address and *zero* routes: no default, and no 205.151.11.13/32,
which is the only path to the MMS proxy. ip route get 205.151.11.13 went out
wlan0. Restored by restarting blueline-clat.service.

nmcli device reapply resets a device to its connection's config, so it is
destructive to precisely the routes a sidecar daemon owns - and the CLAT
daemon owns all of clat's. TASK-49 acceptance #2 says carrier services ride
the carrier; this broke it on the first action it ever took.

The gsm connection was the wrong target too. The modem is v6-only and the v4
default for cellular is installed by the CLAT daemon as
'default dev clat scope link metric 2048', not by the gsm connection, so a
route-metric set there is a number nothing reads. Wifi's metric is the only
lever: under 2048 wifi wins, over it the carrier does. One connection
modified, one device reapplied, and the carrier's routes left alone.

src/sessiond/server.rs

@@ -378,47 +378,65 @@ fn execute(shared: &Arc<Shared>, action: Action) {
     run_executor(shared, program, &args, label, action);
 }
 
-/// Route metrics for the two bearers.
+/// The cellular v4 default's metric, which is a fixed reference point rather
+/// than something to set.
 ///
-/// The numbers are NetworkManager's own defaults for the winning case (wifi
-/// 600), with the loser pushed clear of NM's +20000 connectivity penalty so a
-/// penalised-but-preferred link still beats the demoted one. That penalty is
-/// what inverted the metrics on 2026-07-31: wifi went to 20600 and the clat
-/// default at 2048 won, on a link that could not resolve.
+/// The modem is v6-only; the v4 default for the carrier is installed by the
+/// CLAT daemon on the `clat` tun as `default dev clat scope link metric 2048`,
+/// not by the `gsm` connection. So the *only* lever needed is wifi's metric:
+/// below 2048 it wins, above 2048 the carrier does. Setting anything on the
+/// gsm connection would be adjusting a number nothing reads.
+const CLAT_DEFAULT_METRIC: u32 = 2048;
+
+/// Wifi's metric when it should win: NetworkManager's own default, and
+/// comfortably under the clat default.
 const METRIC_PREFERRED: &str = "600";
+
+/// Wifi's metric when the carrier should win. Above 2048, and clear of NM's
+/// +20000 connectivity penalty so a penalised link cannot accidentally land
+/// back under the CLAT default. That penalty is what inverted things on
+/// 2026-07-31: wifi went to 20600 and the clat default won on a link that
+/// could not resolve.
 const METRIC_DEMOTED: &str = "30000";
 
 /// Tell NetworkManager which link ordinary traffic should take.
 ///
-/// Both connections are modified every time rather than only the winner: the
-/// loser's metric is half the decision, and leaving it at whatever it happened
-/// to be is how the original inversion survived.
+/// **Only the wifi connection is touched, and only `wlan0` is reapplied.** An
+/// earlier version also modified the `gsm` connection and reapplied `clat`, and
+/// that reapply wiped every route the CLAT daemon had installed out-of-band —
+/// including the `205.151.11.13/32` pin that is the only path to the MMS proxy.
+/// Measured on the phone 2026-08-01: after one reapply the clat device was up
+/// with an address and *zero* routes, and the carrier path was gone until
+/// `blueline-clat.service` was restarted. `nmcli device reapply` resets a
+/// device to its connection's config, so it is destructive to exactly the
+/// routes a sidecar daemon owns. Do not reapply a device whose routes NM did
+/// not write.
 fn apply_link_preference(shared: &Arc<Shared>, bearer: crate::sessiond::bearer::Bearer) {
     use crate::sessiond::bearer::Bearer;
-    let (wifi_metric, wwan_metric) = match bearer {
-        Bearer::Wifi => (METRIC_PREFERRED, METRIC_DEMOTED),
-        Bearer::Cellular => (METRIC_DEMOTED, METRIC_PREFERRED),
+    debug_assert!(METRIC_PREFERRED.parse::<u32>().unwrap() < CLAT_DEFAULT_METRIC);
+    debug_assert!(METRIC_DEMOTED.parse::<u32>().unwrap() > CLAT_DEFAULT_METRIC);
+
+    let metric = match bearer {
+        Bearer::Wifi => METRIC_PREFERRED,
+        Bearer::Cellular => METRIC_DEMOTED,
     };
-    for (kind, metric) in [("802-11-wireless", wifi_metric), ("gsm", wwan_metric)] {
-        for conn in active_connections_of_type(kind) {
+    for conn in active_connections_of_type("802-11-wireless") {
         let _ = std::process::Command::new("nmcli")
             .args(["connection", "modify", &conn, "ipv4.route-metric", metric])
             .status();
     }
-    }
     // Reapply rather than up/down. Cycling the connection is what made the old
     // gate's corrections into NM events that re-entered it; `reapply` changes
     // the live config without a state transition, so nothing observing NM sees
     // a link flap.
-    for dev in ["wlan0", "clat"] {
     let _ = std::process::Command::new("nmcli")
-            .args(["device", "reapply", dev])
+        .args(["device", "reapply", "wlan0"])
         .status();
-    }
+
     shared.lock().device_state.record_error(
         "bearer",
         "prefer-link",
-        &format!("preferred {}", bearer.as_str()),
+        &format!("preferred {} (wifi route-metric {metric})", bearer.as_str()),
     );
 }