Add RFC 5424 syslog logging to all service templates

- decnet/logging/syslog_formatter.py: RFC 5424 formatter (local0 facility,
  decnet@55555 SD element ID, full escaping per §6.3.3)
- decnet/logging/file_handler.py: rotating file handler (10 MB / 5 backups),
  path configurable via DECNET_LOG_FILE env var
- templates/decnet_logging.py: combined syslog_line / write_syslog_file /
  forward_syslog helper distributed to all 22 service template dirs
- All templates/*/server.py: replaced ad-hoc JSON _forward/_log with RFC 5424
  syslog_line + write_syslog_file + forward_syslog
- All templates/*/Dockerfile: COPY decnet_logging.py /opt/
- DecnetConfig: added log_file field; CLI: --log-file flag;
  composer injects DECNET_LOG_FILE env var into service containers
- tests/test_syslog_formatter.py + tests/test_file_handler.py: 25 new tests

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

decnet/logging/file_handler.py

@@ -0,0 +1,57 @@
+"""
+Rotating file handler for DECNET syslog output.
+
+Writes RFC 5424 syslog lines to a local file.
+Path is controlled by the DECNET_LOG_FILE environment variable
+(default: /var/log/decnet/decnet.log).
+"""
+
+import logging
+import logging.handlers
+import os
+from pathlib import Path
+
+_LOG_FILE_ENV = "DECNET_LOG_FILE"
+_DEFAULT_LOG_FILE = "/var/log/decnet/decnet.log"
+_MAX_BYTES = 10 * 1024 * 1024  # 10 MB
+_BACKUP_COUNT = 5
+
+_handler: logging.handlers.RotatingFileHandler | None = None
+_logger: logging.Logger | None = None
+
+
+def _get_logger() -> logging.Logger:
+    global _handler, _logger
+    if _logger is not None:
+        return _logger
+
+    log_path = Path(os.environ.get(_LOG_FILE_ENV, _DEFAULT_LOG_FILE))
+    log_path.parent.mkdir(parents=True, exist_ok=True)
+
+    _handler = logging.handlers.RotatingFileHandler(
+        log_path,
+        maxBytes=_MAX_BYTES,
+        backupCount=_BACKUP_COUNT,
+        encoding="utf-8",
+    )
+    _handler.setFormatter(logging.Formatter("%(message)s"))
+
+    _logger = logging.getLogger("decnet.syslog")
+    _logger.setLevel(logging.DEBUG)
+    _logger.propagate = False
+    _logger.addHandler(_handler)
+
+    return _logger
+
+
+def write_syslog(line: str) -> None:
+    """Write a single RFC 5424 syslog line to the rotating log file."""
+    try:
+        _get_logger().info(line)
+    except Exception:
+        pass
+
+
+def get_log_path() -> Path:
+    """Return the configured log file path (for tests/inspection)."""
+    return Path(os.environ.get(_LOG_FILE_ENV, _DEFAULT_LOG_FILE))