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/routers/adventures/nodes.py

@@ -10,22 +10,15 @@ from sqlalchemy.orm import Session, undefer
 
 from ... import attempts, memorybank, models, tree
 from ...context import cursors
-from ...context import history as context_history
 from ...context import lineage
 
 
-def next_index(adventure: models.Adventure) -> int:
-    return context_history.max_action_index(adventure) + 1
-
-
 def next_depth(adventure: models.Adventure) -> int:
     """Returns the depth for the next node played onto this story, one past the tip.
 
-    This is not `next_index`, which returned the same number until SP5. `index`
-    has to stay unique across the whole adventure, because it is the v1 bundle's
-    key. On a story forked at depth 6 after twenty turns, `next_index` gives the
-    next node depth 21 and leaves a fourteen-deep gap in the path. A depth is a
-    position along this one story, and the branch is what makes it unambiguous.
+    A depth is a position along one story, and the branch is what makes it
+    unambiguous. Two branches each hold a node at depth 4, and they are
+    alternatives rather than duplicates.
     """
     return adventure.head_depth + 1