Fix monitor loop double-sleep on error; add grep -F regression test

On exception the monitor slept 30s inside the except block then fell
through to time.sleep(poll_interval), giving a 150s recovery gap instead
of 30s. Adding continue after the error sleep fixes this.

Also adds a regression test asserting dmesg filtering uses grep -F --
so a future refactor cannot silently reintroduce the regex wildcard bug.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

monitor.py

@@ -970,6 +970,7 @@ class NetworkMonitor:
             except Exception as e:
                 logger.error(f'Monitor loop error: {e}', exc_info=True)
                 time.sleep(30)
+                continue
 
             time.sleep(self.poll_interval)
 

tests/test_diagnose.py

@@ -36,6 +36,12 @@ class TestBuildSshCommand:
         cmd = DiagnosticsRunner.build_ssh_command('10.0.0.1', 'eth0')
         assert 'ethtool' in cmd
 
+    def test_dmesg_uses_fixed_string_grep(self):
+        # grep -F prevents iface names with dots (e.g. eth0.1) being treated as
+        # regex wildcards; -- prevents leading - from being parsed as a flag
+        cmd = DiagnosticsRunner.build_ssh_command('10.0.0.1', 'eth0')
+        assert 'grep -F --' in cmd
+
 
 # ── parse_output ─────────────────────────────────────────────────────────────