Validate reconstruction works

Reconstruct text from events; validate we get back original text with:

1. Group editing
2. Complex formatting (images, tables, etc.)
3. Sessions where a student is not using a system with the extension all of the time
4. Etc.

We fail in many of these use cases.

The issue names no files, tests, or entry points, so first locate the text-reconstruction logic and existing event-validation tests. Reproduce the failures for group editing, complex formatting, and sessions with intermittent extension use; done means reconstructed text matches the original across those cases.

Written by the indexing model from the issue text.