test(runtime): verify saveGraph merges to-one and collection relation roots

[philipgreat]

Background

TeaQLRuntime.mergeRelatedEntityRoots recursively merges independently tracked entities into the aggregate root before persistence. Both to-one and collection relations need direct protection.

Scope

Add focused runtime tests using small parent/child entity fixtures and relation metadata. Ledger ordering is tracked separately.

Test cases

• A to-one related entity's changes are merged into the parent root.
• Each entity in a collection relation is merged into the parent root.
• Related entities are rebound to the parent root after merging.
• Nested relations are traversed recursively.
• An entity already sharing the target root is not merged twice.
• Missing descriptors and non-relation properties do not cause failures.

Acceptance criteria

• Tests verify observable mutation requests and shared-root identity.
• Tests do not invoke a real database.
• Parent, child, and nested-child fixtures are kept minimal and local to the test.