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/app/context/history.py

@@ -417,30 +417,6 @@ def newest(adventure: models.Adventure) -> models.Action | None:
     return rows[0] if rows else None
 
 
-def max_action_index(adventure: models.Adventure) -> int:
-    """Highest `Action.index` in the adventure, story text or not. -1 if empty.
-
-    This is the only read in the module that is deliberately not scoped to a
-    path. `index` is a legacy column that remains unread until SP8 drops it. Its
-    one remaining job is to give the next row a number that no other row holds,
-    which is a fact about the adventure rather than about the story being
-    played. Scoping the query to a branch would let two branches issue the same
-    index.
-    """
-    loaded = _loaded_actions(adventure)
-    if loaded is not None:
-        return max((a.index for a in loaded), default=-1)
-    db = _session(adventure)
-    if db is None:
-        return -1
-    highest = (
-        db.query(func.max(models.Action.index))
-        .filter(models.Action.adventure_id == adventure.id)
-        .scalar()
-    )
-    return -1 if highest is None else highest
-
-
 def window_covering(
     adventure: models.Adventure,
     budget_tokens: int,