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/src/ely_eye/migrations.py

@@ -16,7 +16,7 @@ import sqlite3
 from .storage import token_equivalent, visual_token_equivalent
 from .text_index import index_text
 
-SCHEMA_VERSION = 11
+SCHEMA_VERSION = 12
 
 ATOM_COLUMNS_V2 = """
     atom_id TEXT PRIMARY KEY,
@@ -188,7 +188,7 @@ tokenize = 'unicode61'
 VIEWS = """
     CREATE VIEW IF NOT EXISTS v_atoms_current AS
     SELECT * FROM atoms
-    WHERE valid_to IS NULL
+    WHERE (valid_to IS NULL OR valid_to > strftime('%Y-%m-%dT%H:%M:%f+00:00', 'now'))
       AND archived_at IS NULL
       AND (valid_from IS NULL OR valid_from <= strftime('%Y-%m-%dT%H:%M:%f+00:00', 'now'))
       AND (retention_until IS NULL OR retention_until >= strftime('%Y-%m-%dT%H:%M:%f+00:00', 'now'));
@@ -328,8 +328,8 @@ def apply(connection: sqlite3.Connection) -> None:
         for name, definition in _NEW_HOLDER_COLUMNS:
             if name not in existing_holders:
                 connection.execute(f"ALTER TABLE holders ADD COLUMN {name} {definition}")
-    if version < 10:
-        # CREATE VIEW IF NOT EXISTS keeps the old body, and v10 changed which rows are current, so an upgraded store must be told to look again.
+    if version < 12:
+        # CREATE VIEW IF NOT EXISTS keeps the old body, and v10 and v12 each changed which rows are current, so an upgraded store must be told to look again.
         connection.execute("DROP VIEW IF EXISTS v_atoms_current")
 
     connection.executescript(VIEWS)