Drop the eight legacy columns SP8 left behind

Migrations 66 to 73 drop `actions.index`, `variants`, `variant_index`,
`variant_count`, `state_before`, and `world_state_before`, plus
`adventures.memory_cursor` and `summary_cursor`. `index` is a keyword in
SQLite, so migration 71 quotes it.

Nothing outside the migrations read these. `models.py`, `schemas.py`, and
`ACTION_LIST_COLUMNS` lose the same eight fields, `Adventure.actions` orders
by `id`, and `attempts.renumber`, `context.history.max_action_index`, and
`nodes.next_index` are deleted.

Two changes keep the migration replayable on a `create_all` database:

- `_split_variants_into_siblings` wrote through the live ORM table, so it
  stopped compiling once migration 66 removed five of its columns. It now
  writes through `_ACTIONS_AT_60`, a frozen `Table` with its own `MetaData`.
- Five data passes read columns these migrations drop. Each now calls
  `_has_columns` and returns early when the columns are absent.

`bootstrap` takes a `through` version so a migration test can stop at the
schema it asserts on.

555 tests pass, up from 549. Eight of the new cases assert each column is
gone after a real schema-45 database migrates all the way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0198qDK3gmgSo7EtQ4GTPqqK

backend/tests/test_bundle_v2.py

@@ -67,7 +67,7 @@ def client(monkeypatch):
     )
     setup.add(adv)
     setup.flush()
-    setup.add(models.Action(adventure_id=adv.id, index=0, type="start", text=OPENING))
+    setup.add(models.Action(adventure_id=adv.id, type="start", text=OPENING))
     setup.add(models.AdventureScript(
         adventure_id=adv.id, position=0, enabled=True, name="Gold", output_js=GOLD_SCRIPT,
     ))
@@ -163,7 +163,7 @@ def _rows(adv_id) -> list[models.Action]:
             db.query(models.Action)
             .filter(models.Action.adventure_id == adv_id)
             .order_by(models.Action.branch_id, models.Action.depth,
-                      models.Action.variant_index)
+                      models.Action.id)
             .all()
         )
     finally:
@@ -341,29 +341,20 @@ def test_the_lineage_is_rebuilt_rather_than_carried(client):
     assert lineage.entries_of(forked) == [(forked.id, None), (root.id, forked.fork_depth)]
 
 
-def test_the_legacy_index_is_reissued_so_two_branches_never_share_one(client):
-    """`index` describes the adventure. `depth` describes a path.
+def test_two_branches_each_keep_their_own_node_at_one_depth(client):
+    """A depth describes a path, not the adventure.
 
-    Two branches can each have a node at depth 3, so depth cannot be the
-    number `max_action_index` hands out next. The import allocates one
-    index per turn instead. Siblings share an index, the way SP4 leaves
-    them, and no two coordinates share one.
+    The fork and the branch it left both hold a turn at depth 2, and they are
+    not the same turn. The import used to issue a global `index` per turn as
+    well, so that every coordinate had a number nothing else held. SP8 dropped
+    that column, so the coordinate is all there is, and it has to survive the
+    round trip on its own.
     """
     copy = _imported(client, _export(client, _forked_story(client)))
     rows = _rows(copy)
-    by_index: dict[int, set] = {}
-    for row in rows:
-        by_index.setdefault(row.index, set()).add((row.branch_id, row.depth))
-    assert all(len(coords) == 1 for coords in by_index.values()), \
-        "one index per turn, whatever branch it is on"
-    assert len(by_index) == len({(r.branch_id, r.depth) for r in rows})
 
-    # This is the case that makes the rule necessary: the fork and the
-    # branch it left both hold a turn at depth 2, and they are not the
-    # same turn.
     at_depth_2 = [r for r in rows if r.depth == 2]
     assert len({r.branch_id for r in at_depth_2}) == 2
-    assert len({r.index for r in at_depth_2}) == 2, "same depth, different turns"
 
 
 # ------------------------------------------------------- a file that is wrong
@@ -487,7 +478,6 @@ def test_a_v1_bundle_with_a_cursor_lands_it_on_a_node(client):
     db = SessionLocal()
     try:
         adventure = db.get(models.Adventure, copy)
-        assert adventure.memory_cursor == 2, "the legacy count is kept as given"
         assert adventure.memory_cursor_branch_id is not None
         assert adventure.memory_cursor_depth == 1, "the second story action"
     finally:
@@ -495,8 +485,7 @@ def test_a_v1_bundle_with_a_cursor_lands_it_on_a_node(client):
 
 
 def test_a_v2_bundle_brings_its_anchors_back(client):
-    """The other direction: v2 carries the anchor directly, and the legacy
-    count is derived from it."""
+    """The other direction: v2 carries the anchor directly."""
     original = _forked_story(client)
     tip = [a for a in _rows(original) if a.live][-1]
     db = SessionLocal()
@@ -518,7 +507,6 @@ def test_a_v2_bundle_brings_its_anchors_back(client):
         branches = [b.id for b in _branch_rows(copy)]
         assert branches.index(adventure.memory_cursor_branch_id) == 1
         assert adventure.memory_cursor_depth == tip.depth
-        assert adventure.memory_cursor > 0, "the legacy count was read back off it"
     finally:
         db.close()