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_snapshot_compression.py

@@ -95,7 +95,7 @@ def test_unpack_rejects_nothing_it_wrote():
 def test_the_column_stores_bytes_and_returns_a_dict(db, adventure):
     value = snapshot(3)
     action = models.Action(
-        adventure_id=adventure.id, index=0, type="ai", text="t",
+        adventure_id=adventure.id, type="ai", text="t",
         context_snapshot=value,
     )
     db.add(action)
@@ -115,7 +115,7 @@ def test_the_column_stores_bytes_and_returns_a_dict(db, adventure):
 def test_the_column_is_smaller_than_the_json_it_holds(db, adventure):
     value = snapshot(4, 200_000)
     action = models.Action(
-        adventure_id=adventure.id, index=0, type="ai", text="t",
+        adventure_id=adventure.id, type="ai", text="t",
         context_snapshot=value,
     )
     db.add(action)
@@ -130,7 +130,7 @@ def test_the_column_is_smaller_than_the_json_it_holds(db, adventure):
 
 def test_null_stays_null(db, adventure):
     action = models.Action(
-        adventure_id=adventure.id, index=0, type="do", text="t",
+        adventure_id=adventure.id, type="do", text="t",
         context_snapshot=None,
     )
     db.add(action)
@@ -144,7 +144,7 @@ def test_an_unreadable_snapshot_reads_as_none_rather_than_raising(db, adventure)
     it. The snapshot is a debugging view. The story is what actually
     matters."""
     action = models.Action(
-        adventure_id=adventure.id, index=0, type="ai", text="t",
+        adventure_id=adventure.id, type="ai", text="t",
         context_snapshot={"a": "b"},
     )
     db.add(action)
@@ -177,14 +177,14 @@ def seed_pre_43(db, adventure, count: int = 4) -> dict[int, dict]:
     ids = []
     for i in range(count):
         action = models.Action(
-            adventure_id=adventure.id, index=i, type="ai", text=f"t{i}"
+            adventure_id=adventure.id, type="ai", text=f"t{i}"
         )
         db.add(action)
         db.flush()
         ids.append(action.id)
     # One action with no snapshot at all, which must survive as NULL.
     plain = models.Action(
-        adventure_id=adventure.id, index=count, type="do", text="look"
+        adventure_id=adventure.id, type="do", text="look"
     )
     db.add(plain)
     db.commit()