test(overlay): close review gaps in the merge-algebra test suite

Five review findings, all in the test suite the task designates as the
algebra's specification:

- test_diagnostics's headline assertion was vacuous: it checked slug
  presence after an add-over-existing, but hilary was already present
  before the Add ran, so a broken implementation that recorded the
  diagnostic and skipped applying the entry would still pass. Add now
  carries a rank that genuinely differs from base()'s default, and the
  assertion checks the field actually changed.
- Set_subject, Set_name, Remove_name, Set_citation and Remove_citation
  were implemented but never exercised. Added coverage for all five,
  including the case that matters most for Set_citation: setting a part
  that already exists must replace it, not append a duplicate.
- Replace had zero coverage. Added tests for both its existing-slug path
  (entry swapped, no diagnostic) and its absent-slug path (diagnostic
  recorded, entry still added).
- merge's diagnostic accumulation was untested; test_merge_order discarded
  the diagnostics it got back. Added a test asserting two overlays' each
  producing one diagnostic come back in application order.

Extended the cel/entry test helpers with optional subject/names/citations
so the new tests can build fixtures with those fields populated.

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