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/tools/branch_fixture.py

@@ -99,7 +99,7 @@ adv = models.Adventure(
 db.add(adv)
 db.flush()
 db.add(models.Action(
-    adventure_id=adv.id, index=0, type="start",
+    adventure_id=adv.id, type="start",
     text="The cellar door has been shut since your grandmother died. "
          "Tonight the lantern is lit and the key is in your hand."))
 db.add(models.AdventureScript(

backend/tools/stress_session.py

@@ -396,7 +396,7 @@ def add_second_adventure(db, rng: random.Random, user) -> int:
     db.flush()
     for i in range(6):
         action = models.Action(
-            adventure_id=other.id, index=i,
+            adventure_id=other.id,
             type="ai" if i % 2 else "do",
             text=f"[second adventure] turn {i}. {prose(rng, 200)}",
             state_after=rich_script_state(i),
@@ -507,7 +507,6 @@ def build_fixture(args, rng: random.Random) -> tuple[int, int]:
             for n, body in enumerate(texts):
                 action = models.Action(
                     adventure_id=adventure.id,
-                    index=i,
                     type="ai" if is_ai else "do",
                     text=body,
                     # The turn's assembled prompt is stored once, on the

backend/tools/tree_fixture.py

@@ -86,7 +86,7 @@ adv = models.Adventure(
 db.add(adv)
 db.flush()
 db.add(models.Action(
-    adventure_id=adv.id, index=0, type="start",
+    adventure_id=adv.id, type="start",
     text="The cellar door has been shut since your grandmother died."))
 db.commit()
 adv_id = adv.id