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

tests/test_logging.py

@@ -0,0 +1,155 @@
+"""
+Tests for DECNET application logging system.
+
+Covers:
+- get_logger() factory and _ComponentFilter injection
+- Rfc5424Formatter component-aware APP-NAME field
+- Log level gating via DECNET_DEVELOPER
+- Component tags for all five microservice layers
+"""
+
+from __future__ import annotations
+
+import logging
+import os
+import re
+
+import pytest
+
+from decnet.logging import _ComponentFilter, get_logger
+
+# RFC 5424 parser: <PRI>1 TIMESTAMP HOSTNAME APP-NAME PROCID MSGID SD MSG
+_RFC5424_RE = re.compile(
+    r"^<(\d+)>1 "   # PRI
+    r"\S+ "          # TIMESTAMP
+    r"\S+ "          # HOSTNAME
+    r"(\S+) "        # APP-NAME  ← what we care about
+    r"\S+ "          # PROCID
+    r"(\S+) "        # MSGID
+    r"(.+)$",        # SD + MSG
+)
+
+
+def _format_record(logger: logging.Logger, level: int, msg: str) -> str:
+    """Emit a log record through the root handler and return the formatted string."""
+    from decnet.config import Rfc5424Formatter
+    formatter = Rfc5424Formatter()
+    record = logger.makeRecord(
+        logger.name, level, "<test>", 0, msg, (), None
+    )
+    # Run all filters attached to the logger so decnet_component gets injected
+    for f in logger.filters:
+        f.filter(record)
+    return formatter.format(record)
+
+
+class TestGetLogger:
+    def test_returns_logger(self):
+        log = get_logger("cli")
+        assert isinstance(log, logging.Logger)
+
+    def test_logger_name(self):
+        log = get_logger("engine")
+        assert log.name == "decnet.engine"
+
+    def test_filter_attached(self):
+        log = get_logger("api")
+        assert any(isinstance(f, _ComponentFilter) for f in log.filters)
+
+    def test_idempotent_filter(self):
+        log = get_logger("mutator")
+        get_logger("mutator")  # second call
+        component_filters = [f for f in log.filters if isinstance(f, _ComponentFilter)]
+        assert len(component_filters) == 1
+
+    @pytest.mark.parametrize("component", ["cli", "engine", "api", "mutator", "collector"])
+    def test_all_components_registered(self, component):
+        log = get_logger(component)
+        assert any(isinstance(f, _ComponentFilter) for f in log.filters)
+
+
+class TestComponentFilter:
+    def test_injects_attribute(self):
+        f = _ComponentFilter("engine")
+        record = logging.LogRecord("test", logging.INFO, "", 0, "msg", (), None)
+        assert f.filter(record) is True
+        assert record.decnet_component == "engine"  # type: ignore[attr-defined]
+
+    def test_always_passes(self):
+        f = _ComponentFilter("collector")
+        record = logging.LogRecord("test", logging.DEBUG, "", 0, "msg", (), None)
+        assert f.filter(record) is True
+
+
+class TestRfc5424FormatterComponentAware:
+    @pytest.mark.parametrize("component", ["cli", "engine", "api", "mutator", "collector"])
+    def test_app_name_is_component(self, component):
+        log = get_logger(component)
+        line = _format_record(log, logging.INFO, "test message")
+        m = _RFC5424_RE.match(line)
+        assert m is not None, f"Not RFC 5424: {line!r}"
+        assert m.group(2) == component, f"Expected APP-NAME={component!r}, got {m.group(2)!r}"
+
+    def test_fallback_app_name_without_component(self):
+        """Untagged loggers (no _ComponentFilter) fall back to 'decnet'."""
+        from decnet.config import Rfc5424Formatter
+        formatter = Rfc5424Formatter()
+        record = logging.LogRecord("some.module", logging.INFO, "", 0, "hello", (), None)
+        line = formatter.format(record)
+        m = _RFC5424_RE.match(line)
+        assert m is not None
+        assert m.group(2) == "decnet"
+
+    def test_msgid_is_logger_name(self):
+        log = get_logger("engine")
+        line = _format_record(log, logging.INFO, "deploying")
+        m = _RFC5424_RE.match(line)
+        assert m is not None
+        assert m.group(3) == "decnet.engine"
+
+
+class TestLogLevelGating:
+    def test_configure_logging_normal_mode_sets_info(self):
+        """_configure_logging(dev=False) must set root to INFO."""
+        from decnet.config import _configure_logging, Rfc5424Formatter
+        root = logging.getLogger()
+        original_level = root.level
+        original_handlers = root.handlers[:]
+        # Remove any existing RFC5424 handlers so idempotency check doesn't skip
+        root.handlers = [
+            h for h in root.handlers
+            if not (isinstance(h, logging.StreamHandler) and isinstance(h.formatter, Rfc5424Formatter))
+        ]
+        try:
+            _configure_logging(dev=False)
+            assert root.level == logging.INFO
+        finally:
+            root.setLevel(original_level)
+            root.handlers = original_handlers
+
+    def test_configure_logging_dev_mode_sets_debug(self):
+        """_configure_logging(dev=True) must set root to DEBUG."""
+        from decnet.config import _configure_logging, Rfc5424Formatter
+        root = logging.getLogger()
+        original_level = root.level
+        original_handlers = root.handlers[:]
+        root.handlers = [
+            h for h in root.handlers
+            if not (isinstance(h, logging.StreamHandler) and isinstance(h.formatter, Rfc5424Formatter))
+        ]
+        try:
+            _configure_logging(dev=True)
+            assert root.level == logging.DEBUG
+        finally:
+            root.setLevel(original_level)
+            root.handlers = original_handlers
+
+    def test_debug_enabled_in_developer_mode(self, monkeypatch):
+        """Programmatically setting DEBUG on root allows debug records through."""
+        root = logging.getLogger()
+        original_level = root.level
+        root.setLevel(logging.DEBUG)
+        try:
+            assert root.isEnabledFor(logging.DEBUG)
+        finally:
+            root.setLevel(original_level)