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_history_window.py

@@ -66,7 +66,7 @@ def story():
                             entry="A scout with sharp eyes.", type="lore"))
     for i in range(ACTION_COUNT):
         db.add(models.Action(
-            adventure_id=adventure.id, index=i,
+            adventure_id=adventure.id,
             type="ai" if i % 2 else "do",
             text=f"[{i}] {NARRATION}",
             world_delta={"delta": {"player.hp": -1},
@@ -131,7 +131,7 @@ def test_window_matches_on_the_retry_shape(story, monkeypatch):
     last = history.tail(adventure, 1)[0]
 
     windowed = builder.build_context(adventure, settings, exclude_action_id=last.id)
-    assert f"[{last.index}]" not in windowed[1]
+    assert f"[{last.depth}]" not in windowed[1]
 
     monkeypatch.setattr(builder.history, "window_covering", full_window)
     db.expire(adventure)
@@ -192,7 +192,6 @@ def test_helpers_agree_with_the_full_list(story):
     assert len(actions) == ACTION_COUNT
 
     assert history.count(adventure) == len(actions)
-    assert history.max_action_index(adventure) == max(a.index for a in actions)
     assert [a.id for a in history.tail(adventure, 4)] == [a.id for a in actions[-4:]]
     assert [a.id for a in history.slice_(adventure, 10, 6)] == [a.id for a in actions[10:16]]
     assert [a.id for a in history.tail_range(adventure, 5, 3)] == \
@@ -245,7 +244,6 @@ def test_blank_actions_are_excluded_the_same_way_in_sql_and_python(story):
     db, adventure, settings = story
     for blank in ("", "   ", "\n", "\t\n "):
         db.add(models.Action(adventure_id=adventure.id,
-                             index=history.max_action_index(adventure) + 1,
                              type="story", text=blank))
     db.commit()
     db.expire(adventure)