Plugin proposal: al-marimo (from PR #3517)

Source PR: https://github.com/alshedivat/al-folio/pull/3517

This feature does not currently map cleanly to an existing owning plugin.
Under v1 modularization, route this work as a standalone plugin proposal.

Proposal tasks:
- confirm plugin scope and naming for `al-marimo`
- define runtime/test ownership boundaries
- implement in dedicated plugin repo
- submit follow-up PR to `al-folio` only for featuring/bundling metadata if approved