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_state_revert.py

@@ -48,7 +48,7 @@ def _make_adventure(db, script_state):
 
 def _add(db, adv, index, type_, text="x", state_after=None):
     a = models.Action(
-        adventure_id=adv.id, index=index, type=type_, text=text,
+        adventure_id=adv.id, type=type_, text=text,
         state_after=state_after,
     )
     db.add(a)
@@ -186,7 +186,7 @@ def test_forget_node_withdraws_only_what_that_node_produced(db):
 
 def test_snapshot_outcome_is_an_independent_deep_copy(db):
     _, adv = _make_adventure(db, {"nested": {"n": 1}})
-    node = models.Action(adventure_id=adv.id, index=0, type="ai", text="x")
+    node = models.Action(adventure_id=adv.id, type="ai", text="x")
     attempts.snapshot_outcome(adv, node)
     adv.script_state["nested"]["n"] = 99
     assert node.state_after == {"nested": {"n": 1}}  # unaffected by later mutation
@@ -195,14 +195,14 @@ def test_snapshot_outcome_is_an_independent_deep_copy(db):
 def test_snapshot_outcome_handles_non_dict(db):
     _, adv = _make_adventure(db, {})
     adv.script_state = None
-    node = models.Action(adventure_id=adv.id, index=0, type="ai", text="x")
+    node = models.Action(adventure_id=adv.id, type="ai", text="x")
     attempts.snapshot_outcome(adv, node)
     assert node.state_after == {}
 
 
 def test_restore_state_ignores_a_node_with_no_outcome(db):
     _, adv = _make_adventure(db, {"gold": 7})
-    attempts.restore_state(adv, models.Action(adventure_id=adv.id, index=0, type="ai"))
+    attempts.restore_state(adv, models.Action(adventure_id=adv.id, type="ai"))
     assert adv.script_state == {"gold": 7}
     attempts.restore_state(adv, None)
     assert adv.script_state == {"gold": 7}
@@ -236,6 +236,6 @@ def test_retry_restores_the_state_the_turn_started_from(db, monkeypatch):
     assert [a.type for a in adv.actions] == ["start", "do", "ai"]
     last = adv.actions[-1]
     assert last.live is True
-    assert last.variant_count == 0
+    assert len(attempts.group(db, last)) == 1  # no sibling was filed
     assert last.state_after == {"gold": 20}  # its own outcome, untouched
     adventures.turns._active_turns.discard(adv.id)