datagraphs.schema_tracker

Change-tracking and atomic-transaction machinery for schema mutations.

This module owns the subsystem that sits between schema encoding and schema reporting:

• it is the producer of the data datagraphs.schema_report consumes — the baseline snapshot (ChangeTracker.baseline) and the intent-bearing op-log (ChangeTracker.change_log); and
• it makes every public schema mutation all-or-nothing via a footprint-scoped undo journal (ChangeTracker.atomic plus the stage* helpers and the re-entrancy guard ChangeTracker.track).

Separating it from datagraphs.schema.Schema keeps three concerns in three homes: Schema encodes the domain model, ChangeTracker tracks and transacts the edits, and schema_report reports them.

ChangeTracker is a tight collaborator of Schema: rollback restores schema_dict["classes"] in place, so the tracker holds a live reference to the schema dict it guards. Schema constructs a fresh tracker whenever it rebinds that dict (the create_from path), keeping the reference current.