feat: add structured logging configuration module

- Create logging_config.py with configure_logging() helper
- Add tests for level setting, format, idempotency, and inheritance

U1 of P2-001 (replace print with logging)

logging_config.py

@@ -0,0 +1,13 @@
+import logging
+
+DEFAULT_FORMAT = "%(asctime)s - %(name)s - %(levelname)s - %(message)s"
+
+
+def configure_logging(level: int = logging.INFO) -> None:
+    root = logging.getLogger()
+    root.setLevel(level)
+
+    if not root.handlers:
+        handler = logging.StreamHandler()
+        handler.setFormatter(logging.Formatter(DEFAULT_FORMAT))
+        root.addHandler(handler)

tests/test_logging_config.py

@@ -0,0 +1,50 @@
+import logging
+
+import pytest
+
+from logging_config import configure_logging
+
+
+class TestConfigureLogging:
+    def _clear_handlers(self):
+        root = logging.getLogger()
+        root.handlers.clear()
+        root.setLevel(logging.WARNING)
+
+    def test_sets_root_logger_level(self):
+        self._clear_handlers()
+        configure_logging(level=logging.DEBUG)
+        root = logging.getLogger()
+        assert root.level == logging.DEBUG
+
+    def test_default_level_is_info(self):
+        self._clear_handlers()
+        configure_logging()
+        root = logging.getLogger()
+        assert root.level == logging.INFO
+
+    def test_handler_format_includes_name_and_level(self):
+        self._clear_handlers()
+        configure_logging()
+        root = logging.getLogger()
+        assert len(root.handlers) == 1
+        handler = root.handlers[0]
+        formatter = handler.formatter
+        assert "%(name)s" in formatter._fmt
+        assert "%(levelname)s" in formatter._fmt
+        assert "%(asctime)s" in formatter._fmt
+
+    def test_idempotent_second_call(self):
+        self._clear_handlers()
+        configure_logging()
+        initial_handlers = len(logging.getLogger().handlers)
+        configure_logging()
+        assert len(logging.getLogger().handlers) == initial_handlers
+
+    def test_module_logger_inherits_level(self):
+        self._clear_handlers()
+        configure_logging(level=logging.WARNING)
+        logger = logging.getLogger("some.module")
+        assert logger.level == logging.NOTSET
+        assert logger.isEnabledFor(logging.WARNING)
+        assert not logger.isEnabledFor(logging.INFO)