fix: route severe logs to stderr

[kvz]

Why

The full npm-consumer playground run from /tmp/foo showed that severe logs (ALERT/CRIT/ERR/WARN) were going to stdout while verbose logs went to stderr. That split does not match the stream option names or typical CLI behavior.

What changed

• Route WARN and more severe logs/events to stderr.
• Keep NOTICE, INFO, DEBUG, and TRACE on stdout.
• Clarify the public stream option comments.
• Add regression coverage for formatted log and event stream routing.
• Version @transloadit/sev-logger to 1.0.4 with a changelog entry.

Verification

• corepack yarn install --immutable
• corepack yarn dedupe --check
• corepack yarn check
• corepack yarn npm audit --recursive --all
• git diff --check