test(core): cover EntityRoot new and delete lifecycle semantics

[philipgreat]

Background

EntityRoot owns new/deleted key sets and coordinates pending field changes. These lifecycle rules are not directly tested.

Scope

Add focused EntityRootTest cases for new and deleted entities. Root merging is tracked separately.

Test cases

• markAsNew records only the requested key.
• Repeating markAsNew is idempotent.
• newKeys() is read-only.
• markAsDelete records the requested key.
• Repeating markAsDelete is idempotent.
• Marking an entity deleted clears that entity's pending changes from all change-set scopes.
• Deleting one entity does not clear changes for another entity.
• deletedKeys() is read-only.

Acceptance criteria

• Tests require no metadata factory or runtime.
• Lifecycle assertions are scoped to one behavior per test method.