Expand bootstrap/repair parenthetical in Fundamentals topic 19

[rustyrazorblade]

In plugins/cassandra-expert/references/training/01-fundamentals/19-compaction-overview.md:56, the parenthetical that forward-references bootstrap and repair is asymmetric: repair gets a short definition in context but bootstrap doesn't.

Current:

(Bootstrap and repair aren't covered in detail in this training yet — for now, think of both as operator-triggered processes that move data between nodes. See Repair and Streaming for the full references.)

Proposed tightening:

(Bootstrap and repair aren't covered in detail in this training yet — for now, think of bootstrap as adding a new node to the ring and streaming its assigned token ranges from existing replicas, and repair as periodically reconciling data between replicas to fix any drift. See Repair and Streaming for the full references.)

Flagged by PR #2 review on 2026-04-13. Deferred — not blocking.