Feature request: optionally merge stdout and stderr

Sometimes it's really helpful to have stdout and stderr in the same window. I would love to have them optionally mixed together with color-coding for each.

The issue names no files, tests, or entry points. Start by locating the TUI code that renders stdout and stderr, then trace how each stream reaches its window. Done means an optional mode displays both streams together with distinct color-coding; add or update coverage for that behavior if the project has relevant tests.

Written by the indexing model from the issue text.