Provide visual feedback on evaluation progress

Right now it's really hard to tell what's going on when one has long-running computation in their notebook ("is it still running, or did the server die?"). We should have a live-updating marker next to each compilation unit that informs about its evaluation status (queued/in progress/evaluated/errored).

The issue names no files, tests, or entry points, so first locate the notebook evaluation flow and the rendering of compilation units. Define and verify a live marker that distinguishes queued, in-progress, evaluated, and errored states during long-running computation.

Written by the indexing model from the issue text.