Make a retry a node, not a rewrite

Every attempt at a turn is now its own row at the same (branch, depth),
with `live` naming the one the story tells. The JSON repeating group on
`actions.variants` is read one last time, by a migration that writes it
out as the sibling rows it always described, and then goes unread.

The snapshots turn around with it: an action carries the state it left
behind rather than the state it started from, because attempts at one
turn share a starting position and differ exactly in their outcome.
Rolling back is "what the node in front left behind", one lookup on the
path, and it is what undo and retry now both read.

And the memory holdback goes. It existed because retry rewrote a row
under a mark that had already moved past it; a retry writes a sibling
now, and replacing what a coordinate says withdraws what was derived
from it — the same repair undo and delete already made.

The assembled prompt is still stored once per turn: it moves with the
live flag, so a superseded attempt keeps only the few hundred bytes that
were its own. Measured on the 600-action fixture: 700 rows for the same
600-turn story, prompt archive byte-identical at 0.50 MB, index 1.8 kB
and page load 62.7 kB unmoved.

347 tests green. `tests/test_story_tree_baseline.py` and
`tests/test_retry_variants.py` pass unmodified — SP4 was allowed to move
the baseline for the variant-count semantics and did not need to.

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

backend/tests/test_egress.py

@@ -145,12 +145,17 @@ def test_loading_an_adventure_does_not_fetch_context_snapshot(client, sql_log):
     assert offenders == [], f"context_snapshot was fetched in bulk:\n{offenders[0][:400]}"
 
 
-def test_state_before_and_world_state_before_are_not_fetched_in_bulk(client, sql_log):
-    """Both are rollback snapshots, only ever needed for the single action
-    being undone or retried."""
+def test_the_state_snapshots_are_not_fetched_in_bulk(client, sql_log):
+    """All four are rollback snapshots, only ever needed for the single node
+    being undone, retried past or switched to.
+
+    The `_after` pair is the live one since SP4 and the `_before` pair is dead
+    weight until SP8 drops it — a page load must pay for neither.
+    """
     client.get(f"/api/adventures/{client.adv_id}")
     selects = action_selects(sql_log)
-    for column in ("state_before", "world_state_before"):
+    for column in ("state_before", "world_state_before",
+                   "state_after", "world_state_after"):
         offenders = [s for s in selects if column in s]
         assert offenders == [], f"{column} was fetched in bulk"
 
@@ -199,7 +204,8 @@ def test_counting_actions_does_not_name_the_deferred_columns(client, sql_log):
         assert history.count(adventure) == 12
         counts = [s for s in sql_log if "count" in s.lower()]
         assert counts, "expected a COUNT to be emitted"
-        for column in ("context_snapshot", "state_before", "world_state_before", "variants"):
+        for column in ("context_snapshot", "state_after", "world_state_after",
+                       "state_before", "world_state_before", "variants"):
             assert not any(column in s for s in counts), (
                 f"{column} is named by the count query:\n{counts[0][:400]}"
             )