Group a turn's takes by their parent, not by where they sit

SP7 shipped a tree nobody could use. Driving it by hand found three things,
and the schema is what the third one needs.

A take forked onto its own branch leaves the coordinate its siblings are still
at. `attempts.group` filtered on (branch_id, depth), so that take read as the
only one of its turn -- 1/1 where the player is owed 1/3, with the other takes
unreachable from the line they were taken on. Nesting has the same shape from
the other side: takes under C1 and takes under C2 share a depth, and only the
parent says a pager under C2 reads 2/2 rather than counting C1's three too.

So `actions.parent_id`, and `group` keys on it. The alternative -- pointing a
branch's fork at a node instead of a depth, so a promoted take never moves --
was rejected: `lineage` exists so a read is an OR-clause per branch rather
than a walk up parent pointers, and moving the fork point changes path
resolution itself, which drags in cursors, memory depths and both bundle
formats. This column is read to group takes and for nothing else. One indexed
lookup, never a walk, and no read of the story changes.

Two writers had to learn it. `add_attempt` places a sibling by hand rather
than through `place_action`, and copies the parent, because a take belongs to
the turn it is a take *of*. `place_action` resolves it from the path for a
genuinely new node, which is the honest default.

And one reader had to be kept out of it. `delete_turn` meant "every attempt at
this coordinate"; the group spans branches now, so undoing a turn would have
deleted a take that another branch is telling. `attempts.on_branch` scopes it
back.

409 tests, unchanged from main and green -- a linear story has one take per
parent, so none of this is reachable until a second one exists. The migration
adds one column, backfills the linear case, and leaves NULL where it cannot
honestly place a row; `group` falls back to the coordinate there, which is the
rule those rows were written under. One `VACUUM FULL actions;` owed after
deploy.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Dvvqn9ZDR4ixeFPHNbww7

backend/app/routers/adventures.py

@@ -1047,7 +1047,10 @@ def delete_turn(
     rather than off one of its attempts.
     """
     memorybank.forget_node(db, adventure, node)
-    for attempt in attempts.group(db, node):
+    # Scoped to this node's branch (SP9). The group spans branches now, and a
+    # take that was forked onto its own line is another branch's story — see
+    # `attempts.on_branch`.
+    for attempt in attempts.on_branch(attempts.group(db, node), node):
         db.delete(attempt)