test(core): define explicit-null shadowing behavior in ChangeSetStack

[philipgreat]

Background

ChangeSetStack.get() currently treats a returned null as if a field were absent and continues searching lower scopes. That makes it impossible to distinguish an explicit update-to-null from no update in the current scope.

Scope

Add a regression test that defines the required behavior for explicit null values in nested scopes, and make the smallest production correction needed for the test to pass.

Test cases

• A lower scope stores a non-null field value.
• A higher scope explicitly stores null for the same entity and field.
• Reading while the higher scope is active returns null, not the lower value.
• Popping the higher scope restores the lower value.
• An entirely absent field still returns null.

Acceptance criteria

• Presence and value are distinguished internally without changing the public API unnecessarily.
• Existing non-null stack-precedence behavior remains unchanged.
• A regression test fails against the current implementation and passes with the fix.