Let any turn be played again, not just the newest

`retry` only ever saw the last action: mid-story there was no way to ask for
another take at all. Now the take endpoint takes either kind of node, and the
kind decides what happens -- an AI turn regenerates, a player turn takes the
text supplied. The client asks the same way for both.

The tip is the only case that needs no branch, and only for an AI turn, where
the takes are still leaves nobody has built on. That is the existing retry,
reached by another road. Everywhere else `branch_at` leaves the path just
before the turn so the story after it keeps the take it was written for.

Both paths roll the shared state back to before the turn ran, which the fork
endpoint was already doing and the player-take path was not -- a script would
otherwise have stacked this take's output mutations on the one being replaced.

426 tests.

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

backend/tests/test_take_parentage.py

@@ -358,12 +358,47 @@ def test_both_takes_of_a_players_turn_are_one_group(client):
     assert _group_size(first.id) == 2, "and reads the same from the other take"
 
 
-def test_an_ai_turn_is_refused_by_the_take_endpoint(client):
+def test_an_ai_turn_at_the_tip_takes_no_branch(client):
+    """Its takes are still leaves. This is `retry`, reached the other way."""
     _play(client)
+    before = _branch_count(client.adv_id)
+
     ai = _ai_rows(client.adv_id)[0]
-    r = _take(client, ai.id, "nope")
-    assert r.status_code == 400
-    assert "Retry" in r.json()["detail"]
+    r = _take(client, ai.id, "")
+    assert r.status_code == 200, r.text
+
+    assert _branch_count(client.adv_id) == before
+    assert len(_ai_rows(client.adv_id)) == 2, "a second take, beside the first"
+
+
+def test_an_ai_turn_the_story_moved_past_takes_a_branch(client):
+    """Retry could never reach here at all — it only ever saw the newest turn."""
+    _play(client)
+    _play(client, "press on")
+    before = _branch_count(client.adv_id)
+    first_ai = _ai_rows(client.adv_id)[0]
+
+    r = _take(client, first_ai.id, "")
+    assert r.status_code == 200, r.text
+
+    assert _branch_count(client.adv_id) == before + 1
+    # The turn the story moved past now has two takes, and the line it was on
+    # keeps the one it was written for.
+    assert _group_size(first_ai.id) == 2
+    assert first_ai.id in {a.id for a in _ai_rows(client.adv_id)}
+
+
+def test_the_old_line_still_has_its_continuation(client):
+    _play(client, "open the door")
+    _play(client, "press on")
+    first_ai = _ai_rows(client.adv_id)[0]
+    _take(client, first_ai.id, "")
+
+    # The new take is what this branch tells; "press on" belonged to the other.
+    blob = "\n".join(_path_texts(client))
+    assert "press on" not in blob
+    kept = "\n".join(a.text for a in _user_rows(client.adv_id))
+    assert "press on" in kept, "still there, on the line it was played on"
 
 
 def test_the_opening_has_no_other_take(client):