test(core): cover ChangeSetStack nested-scope precedence and pop fallback

[philipgreat]

Background

ChangeSetStack supports nested save scopes, but no unit test currently protects stack precedence.

Scope

Add focused tests for non-null values in nested scopes. Explicit-null behavior is tracked separately.

Test cases

• current() and pop() return null for an empty stack.
• The first set() lazily creates the current change set.
• A value in the top scope shadows the same field in a lower scope.
• Popping the top scope restores visibility of the lower value.
• clearCurrent() clears only the top scope.
• changedFieldNames(key) returns the union of fields across scopes.
• clearEntity(key) removes the entity from every scope without affecting other entities.

Acceptance criteria

• Tests use only ChangeSetStack, EntityChangeSet, and EntityKey.
• Nested-scope behavior is verified without reflection into the stack implementation.