Internal logging within the kernel is a bit of a mess. There's a combination of java.util.logging (JUL), System.out, System.err, and SLF4J (via libraries; we kinda handled this one per #99 , routing it to JUL ) and no configuration for JUL. So...

1. Switch all loggers (including System.out, System.err) in the kernel to SLF4J. We already have it in the stack anyways, might as well use it.
2. Route to JUL at the very last moment (in jjava-distro, and also in jjava-launcher)
3. Bundle a JUL config that is compatible with the default Jupyter log format (single line, labeled as [I 2025-10-19 17:03:44.292 logger_name]
4. Unify logging everywhere (one exception - still keeping System.err in magics, as this will send logs to the notebook, as we'd expect)
5. Adjust logging levels in the core framework for less noise