log: split streams so levels below WARNING go to stdout

[rezib]

Summary

rfl.log.setup_logger currently attaches a single logging.StreamHandler() with no stream argument, so all accepted records go to stderr.

For container/CLI usage it is useful to follow the common split:

• levels below WARNING (DEBUG, INFO) → stdout
• WARNING and above → stderr

That keeps operational noise off stderr (so tools that treat stderr as “errors only” stay quiet) while preserving WARNING/ERROR/CRITICAL on stderr.

Current behavior

In setup_logger, after resolving the root level and flag filter, RFL does roughly:

handler = logging.StreamHandler()  # defaults to sys.stderr
handler.setLevel(logging_level)
# … formatter + component flag filter …
root_logger.addHandler(handler)

There is no API to choose streams or to split by level.

Proposed behavior

When enabled (exact API TBD), setup_logger would configure two stream handlers sharing the existing formatter and log_flags / debug_flags filter:

Handler stream	Records
sys.stdout	levelno < logging.WARNING (and still gated by the configured level)
sys.stderr	levelno >= logging.WARNING (handler level at least WARNING)

Configured level / debug semantics and component flag filtering should stay unchanged; only the destination stream would change.

Suggested API

Backward-compatible options, e.g.:

• split_streams: bool = False (default keeps today’s single-stderr behavior), or
• stream=... plus an optional error_stream=... / threshold

Default should remain current behavior so existing callers are not surprised.