Address Sourcery review: subprocess audit, dead code, renderer fix

- clock/data.py: add nosemgrep suppression on audited _cli subprocess call
- clock/renderer.py: remove unreachable zen-mode check in heartbeat()
- nodeconnection.py: extract _run_ln helper with nosemgrep suppression
- SPV/spvblock.py: add nosemgrep suppression on audited subprocess calls

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pybitblock/clock/data.py

@@ -78,6 +78,7 @@ class ClockData:
     def _cli(self, command):
         """Run bitcoin-cli command, return stdout string."""
         cmd = shlex.split(self.path["bitcoincli"]) + shlex.split(command)
+        # nosemgrep: python.lang.security.audit.dangerous-subprocess-use-audit
         result = subprocess.run(cmd, capture_output=True, text=True)
         return result.stdout.strip()