failover: fix enforce_route() to handle gateway-less WANs (cellular)

The EC25/Koodo GSM connection is QMI raw-ip with an on-link /29 and no
gateway at all (nh 0.0.0.0), not a normal DHCP WAN with a temporarily
unknown gateway. enforce_route() required a gw and silently gave up
when none was found, so cellular could be reported up/healthy/preferred
in state.json while never actually getting a default route (Prefer
button had no effect since it only touches metric, not this bailout).
Now falls back to installing a gateway-less `ip route ... dev DEV`
default when the device genuinely has an IPv4 address (carrier truly
up), mirroring what NM itself would install without never-default.
Also fixes stale-route pruning to handle gateway-less old routes,
which the previous gw-only check skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

failover/van-failover

@@ -117,6 +117,28 @@ def routes_on(dev):
     return out
 
 
+def _del_default(dev, r):
+    args = ["ip", "route", "del", "default", "dev", dev, "metric", str(r["metric"])]
+    if r["gw"]:
+        args[4:4] = ["via", r["gw"]]
+    sh(args)
+
+
+def has_ipv4(dev):
+    """True if dev currently carries an IPv4 address (carrier genuinely up), used as the
+    signal to install a gateway-less default route for point-to-point/on-link WANs (e.g.
+    the EC25 modem's QMI raw-ip /29, which has no gateway at all — nh 0.0.0.0 — unlike
+    a normal DHCP WAN whose gateway is just temporarily unknown)."""
+    r = sh(["ip", "-4", "-j", "addr", "show", "dev", dev])
+    if not (r and r.stdout.strip()):
+        return False
+    try:
+        data = json.loads(r.stdout)
+        return bool(data and data[0].get("addr_info"))
+    except (json.JSONDecodeError, IndexError):
+        return False
+
+
 def enforce_route(dev, metric, conn=None):
     """Ensure exactly one default route on dev at the desired metric, via `ip route`.
     NEVER use `nmcli device reapply` — it resets r8152 USB-ethernet carriers and causes
@@ -127,23 +149,26 @@ def enforce_route(dev, metric, conn=None):
     if any(r["metric"] == metric for r in rts):
         # desired metric already present; just prune any stale others
         for r in rts:
-            if r["metric"] != metric and r["gw"]:
-                sh(["ip", "route", "del", "default", "via", r["gw"], "dev", dev, "metric", str(r["metric"])])
+            if r["metric"] != metric:
+                _del_default(dev, r)
         return
     # Prefer the gw from an existing default route; fall back to NM's known gateway so we can
     # also *restore* a route that went missing while the carrier is still up (not just rebase one).
     gw = next((r["gw"] for r in rts if r["gw"]), None) or nm_gateway(dev, conn)
-    if not gw:
-        return  # no gateway known (carrier down); profile metric still set
-    r = sh(["ip", "route", "add", "default", "via", gw, "dev", dev, "metric", str(metric), "proto", "static"])
+    args = ["ip", "route", "add", "default", "dev", dev, "metric", str(metric), "proto", "static"]
+    if gw:
+        args[4:4] = ["via", gw]
+    elif not has_ipv4(dev):
+        return  # no gateway known and carrier not actually up; nothing to route via
+    r = sh(args)
     if not (r and r.returncode == 0):
         # Add failed — most likely another dev transiently holds this exact metric during a
         # preference swap. Leave the existing route intact and retry next loop; do NOT prune,
         # or we'd strand this dev with no default route at all.
         return
     for old in rts:
-        if old["metric"] != metric and old["gw"]:
-            sh(["ip", "route", "del", "default", "via", old["gw"], "dev", dev, "metric", str(old["metric"])])
+        if old["metric"] != metric:
+            _del_default(dev, old)
 
 
 def nm_gateway(dev, conn):