Build a lightweight, rebuildable document context index #42

Goal

Represent the current document compactly so later requests can select relevant context without replaying everything.

Depends on

• Measure AI latency, token use, repairs, and edit isolation #34 for measurement.

Scope

Derive and maintain:

• section boundaries and headings;
• stable block IDs and short summaries;
• table, chart, and diagram component IDs;
• graph relationships;
• recent touched blocks.

The document remains the source of truth. The index must be disposable and rebuildable; do not introduce a vector database.

Acceptance criteria

• The index updates after insert, update, remove, move, undo, revert, and document load.
• Stale IDs cannot survive a rebuild.
• Index generation is deterministic and covered for large documents.
• No user document data is stored outside the existing document lifecycle merely for this index.