Let a retired memory keep its vector and its place in the present

The second review round of p2-4 measured three behaviour defects behind the
as_of feature it had just accepted, and all three came from one mistake: the
code asked "is this memory current?" in three places and answered differently.

- A memory that has closed kept no vector at all, so the dense leg could only
  exclude an old fact, never recall it. Vectors now follow existence, not
  currency; `_allowed` decides who may see one.
- "Open right now" meant `valid_to IS NULL` in the view and in the words leg
  but `valid_to IS NULL OR valid_to > now` in the vector leg, so a supersede
  booked for next year hid a fact that is still true today. All three now read
  the same rule; schema v12 records the change and the upgrade drops the old
  view, since CREATE VIEW IF NOT EXISTS would have kept the old body.
- The dense as_of test was名不副实: dense is off by default, so it passed while
  the vector side had no coverage. It now turns dense on with a stand-in model
  and pins both `VectorIndex.search(as_of=…)` and the plumbing in retrieval.

Five mutations, each caught by exactly one test: the dense as_of branch, the
retrieval hand-off, the three vector reads, the words-leg currency, the view.

Record side: the round-2 line quoted a D-list I had misread out of the
reviewer's PASS tables (cf2de31 fixed those, and they were its precision notes,
not its findings), the stale line-number parenthetical, an acceptance grep that
scanned CHECKLIST's own quotations, and a Gaps.md tally that disagreed with its
own table (8 closed, 3 closed with reservations, 1 partial).

Gates: 411 passed; ruff, filesize, audit 33/2/6 clean; pyright 0 on the six
touched files; mypy cold cache Success on the same six; scripts/key_test.sh
exit 0 (6 PASS, 1 NOT MEASURED).

backend/tests/test_vector_index.py

@@ -225,17 +225,23 @@ def test_the_index_survives_a_reopen(tmp_path: Path, fake_embeddings: Callable[[
     assert reopened.search(reopened.embeddings.embed_query(KEYS), 10)
 
 
-def test_superseding_moves_the_vector_rather_than_keeping_two(tmp_path: Path, fake_embeddings: Callable[[], Any]) -> None:
+def test_a_superseded_memory_keeps_its_vector_but_leaves_the_current_answer_alone(
+    tmp_path: Path, fake_embeddings: Callable[[], Any]
+) -> None:
     settings, database = _settings(tmp_path), Database(_settings(tmp_path))
     index = _index(settings, database, fake_embeddings)
     memory = _memory(settings, database, index)
     old = memory.remember(KEYS, holder_id="wife", origin=MemoryOrigin.self_)
 
-    memory.supersede(old.atom_id, text=PASSPORT, holder_id="wife")
+    replacement = memory.supersede(old.atom_id, text=PASSPORT, holder_id="wife")
 
     facts = index.stats()
-    assert facts["vectors"] == 1, "the closed memory lost its vector and the replacement took its place"
+    assert facts["vectors"] == 2, "a closed memory keeps its vector, or a question about the past cannot reach it"
     assert facts["pending"] == 0
+    current = index.search(index.embeddings.embed_query(KEYS), 10, as_holder="wife")
+    assert list(current) == [replacement.atom_id], "asked now, only the replacement answers"
+    then = index.search(index.embeddings.embed_query(KEYS), 10, as_holder="wife", as_of=old.ingested_at.isoformat())
+    assert list(then) == [old.atom_id], "asked before the retirement, the old memory is still findable"
 
 
 def test_a_store_that_never_built_its_index_says_so_instead_of_stalling_a_search(