Use subsystem recovery for Sargo modem faults

Select RELATED before HAL startup and create oFono’s private state directory. A controlled modem restart recovered network registration without rebooting the phone; two persistent boots and native UI checks passed. The IMS firmware watchdog root cause remains unresolved.

tundra/tests/test_modem_recovery.py

@@ -0,0 +1,42 @@
+# SPDX-License-Identifier: MPL-2.0
+"""Check modem selection without modifying a host's subsystem policy."""
+import importlib.util
+from pathlib import Path
+import tempfile
+import unittest
+
+spec = importlib.util.spec_from_file_location(
+    'modem_recovery', Path(__file__).resolve().parents[1]/'runtime/modem-recovery.py')
+recovery = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(recovery)
+
+
+class ModemRecoveryTest(unittest.TestCase):
+    def test_only_modem_policy_changes_and_repeated_setup_is_safe(self):
+        with tempfile.TemporaryDirectory() as folder:
+            root = Path(folder)
+            for identifier, name in [('subsys1', 'adsp'), ('subsys7', 'modem')]:
+                device = root/identifier
+                device.mkdir()
+                (device/'name').write_text(name+'\n')
+                (device/'restart_level').write_text('SYSTEM\n')
+            recovery.configure(root)
+            recovery.configure(root)
+            self.assertEqual((root/'subsys1/restart_level').read_text(), 'SYSTEM\n')
+            self.assertEqual((root/'subsys7/restart_level').read_text(), 'RELATED\n')
+
+    def test_missing_modem_is_reported(self):
+        with tempfile.TemporaryDirectory() as folder:
+            with self.assertRaisesRegex(RuntimeError, 'Expected one'):
+                recovery.configure(Path(folder))
+
+    def test_unknown_policy_is_preserved(self):
+        with tempfile.TemporaryDirectory() as folder:
+            root = Path(folder)
+            device = root/'subsys0'
+            device.mkdir()
+            (device/'name').write_text('modem\n')
+            (device/'restart_level').write_text('UNKNOWN\n')
+            with self.assertRaisesRegex(RuntimeError, 'Unsupported'):
+                recovery.configure(root)
+            self.assertEqual((device/'restart_level').read_text(), 'UNKNOWN\n')