test(memory): add direct unit tests for edges_at_timestamp, edge_history, bfs_at_timestamp

[bug-ops]

Missing store-level unit tests for three new public methods added in #1775:

• edges_at_timestamp(): no boundary tests (valid_from==ts, valid_to==ts, open-ended active edges)
• edge_history(): zero tests
• bfs_at_timestamp(): only exercised indirectly through graph_recall

Add direct unit tests covering boundary conditions and happy paths for each method.

Found by: tester validation in feat/issue-1341.