test(core): cover EntityChangeSet field mutation and read-only views

[philipgreat]

Background

EntityChangeSet is the smallest mutable unit in the entity ledger and currently has no direct tests.

Scope

Add a focused EntityChangeSetTest.

Test cases

• A new change set is empty.
• Setting a field makes it readable by entity key and field name.
• Setting the same field twice keeps the latest value.
• Changes for two entity keys remain isolated.
• fieldNames(key) returns only fields changed for that key.
• clearEntity(key) removes only the target entity's changes.
• The map returned by changes() cannot be structurally modified.
• The set returned by fieldNames() cannot be modified.

Acceptance criteria

• Tests are deterministic and require no runtime or database fixture.
• Each assertion describes one public EntityChangeSet contract.