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_branch_forking.py

@@ -59,7 +59,7 @@ def client(monkeypatch):
     )
     setup.add(adv)
     setup.flush()
-    setup.add(models.Action(adventure_id=adv.id, index=0, type="start", text="You enter a cave."))
+    setup.add(models.Action(adventure_id=adv.id, type="start", text="You enter a cave."))
     setup.add(models.AdventureScript(
         adventure_id=adv.id, position=0, enabled=True, name="Gold", output_js=GOLD_SCRIPT,
     ))
@@ -132,7 +132,7 @@ def _rows(adv_id) -> list[models.Action]:
         return (
             db.query(models.Action)
             .filter(models.Action.adventure_id == adv_id)
-            .order_by(models.Action.depth, models.Action.variant_index)
+            .order_by(models.Action.depth, models.Action.id)
             .all()
         )
     finally:
@@ -233,7 +233,6 @@ def test_the_parent_keeps_a_live_attempt_where_the_fork_left(client):
         # offers a page through attempts that diverged onto another branch.
         parent_turn = per_coordinate[(parent_id, 2)]
         assert len(parent_turn) == 1
-        assert parent_turn[0].variant_count == 0
     finally:
         db.close()