test(core): cover BaseEntity root-backed property change tracking

[philipgreat]

Background

BaseEntity.handleUpdate, __internalGet, getUpdatedProperties, and dirtyFields now interact with EntityRoot. This path has no direct unit tests.

Scope

Create a minimal test entity with one typed property and test only root-backed property tracking.

Test cases

• Updating a persisted entity records the new value under its EntityKey.
• Generic reads see the latest value recorded in the root.
• getUpdatedProperties() and dirtyFields() report the changed field.
• Repeating an update with the same value does not add a new dirty field.
• Returning a property to its original value removes it from the entity-local change view.
• A trace chain present during update is copied to the root.
• An entity without an ID falls back to its local change-tracking behavior.

Acceptance criteria

• The test fixture overrides only the supported framework-internal accessors.
• Tests remain inside teaql-core and require no database.
• Any discrepancy between local and root-backed dirty state is documented by a failing test before production code is changed.