feat: structured DEBUG/INFO logging via loguru

- common/logging.py: improve format to timestamp|LEVEL|module:func:line - message
- core/ layer: DEBUG log every subprocess invocation (cmd, rc, byte counts),
  JSON load/dump events, parsed application_id. ERROR log on failures.
- tools/ layer: DEBUG log every public tool entry with key parameters,
  INFO log on business outcomes (saved/submitted/killed/...).
- New tests/unit/test_logging.py: capture loguru output via in-memory sink
  and assert DEBUG + INFO messages are emitted for representative flows.

spark_executor/core/log_parser.py

@@ -5,6 +5,8 @@
 """
 import re
 
+from common.logging import logger
+
 _APP_ID_RE = re.compile(r"Submitted application (\S+)")
 _TRACKING_URL_RE = re.compile(r"tracking URL:\s+(\S+)")
 
@@ -20,14 +22,20 @@ def parse_spark_submit_output(stderr: str) -> tuple[str, str | None]:
     else:
         url_match = _TRACKING_URL_RE.search(stderr)
         if not url_match:
+            logger.error("Could not find application_id in spark-submit output")
             raise ValueError("Could not find application_id in spark-submit output")
         # tracking URL is of the form http://rm:8088/proxy/application_xxx/
         tracking_url = url_match.group(1)
         tail = tracking_url.rstrip("/").rsplit("/", 1)[-1]
         if not tail.startswith("application_"):
+            logger.error("Could not find application_id in spark-submit output (bad tracking URL)")
             raise ValueError("Could not find application_id in spark-submit output")
         application_id = tail
 
     url_match = _TRACKING_URL_RE.search(stderr)
     tracking_url = url_match.group(1) if url_match else None
+    logger.debug(
+        f"parse_spark_submit_output -> application_id={application_id} "
+        f"tracking_url={tracking_url}"
+    )
     return application_id, tracking_url