display whether a line comes from stdout or stderr

This could for example be a single colored character left of the line.

Would somebody be interested in this feature ?

The issue names no file, test, or entry point. Start by locating bacon's terminal output-rendering path and how stdout and stderr lines are handled; define the indicator and color behavior, then verify that displayed lines clearly identify their source without disrupting existing output.

Written by the indexing model from the issue text.