feat: add component-aware RFC 5424 application logging system

- Modify Rfc5424Formatter to read decnet_component from LogRecord
  and use it as RFC 5424 APP-NAME field (falls back to 'decnet')
- Add get_logger(component) factory in decnet/logging/__init__.py
  with _ComponentFilter that injects decnet_component on each record
- Wire all five layers to their component tag:
    cli -> 'cli', engine -> 'engine', api -> 'api' (api.py, ingester,
    routers), mutator -> 'mutator', collector -> 'collector'
- Add structured INFO/DEBUG/WARNING/ERROR log calls throughout each
  layer per the defined vocabulary; DEBUG calls are suppressed unless
  DECNET_DEVELOPER=true
- Add tests/test_logging.py covering factory, filter, formatter
  component-awareness, fallback behaviour, and level gating

decnet/logging/__init__.py

@@ -0,0 +1,42 @@
+"""
+DECNET application logging helpers.
+
+Usage:
+    from decnet.logging import get_logger
+    log = get_logger("engine")   # APP-NAME in RFC 5424 output becomes "engine"
+
+The returned logger propagates to the root logger (configured in config.py with
+Rfc5424Formatter), so level control via DECNET_DEVELOPER still applies globally.
+"""
+
+from __future__ import annotations
+
+import logging
+
+
+class _ComponentFilter(logging.Filter):
+    """Injects *decnet_component* onto every LogRecord so Rfc5424Formatter can
+    use it as the RFC 5424 APP-NAME field instead of the hardcoded "decnet"."""
+
+    def __init__(self, component: str) -> None:
+        super().__init__()
+        self.component = component
+
+    def filter(self, record: logging.LogRecord) -> bool:
+        record.decnet_component = self.component  # type: ignore[attr-defined]
+        return True
+
+
+def get_logger(component: str) -> logging.Logger:
+    """Return a named logger that self-identifies as *component* in RFC 5424.
+
+    Valid components: cli, engine, api, mutator, collector.
+
+    The logger is named ``decnet.<component>`` and propagates normally, so the
+    root handler (Rfc5424Formatter + level gate from DECNET_DEVELOPER) handles
+    output. Calling this function multiple times for the same component is safe.
+    """
+    logger = logging.getLogger(f"decnet.{component}")
+    if not any(isinstance(f, _ComponentFilter) for f in logger.filters):
+        logger.addFilter(_ComponentFilter(component))
+    return logger