A rewound turn's id is forgotten, so sending it again runs the turn anew

The turn log answered a repeated id with what that turn had answered, which
was right for a prompt sent twice over a dropped link and wrong for a turn
the web UI had just rewound to edit: the same id came back with the old
reply. rewind now returns the ids it took out, both handlers forget them from
the log, and the turn file drops them too, or the next load would have
brought them back.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

CHANGELOG.md

@@ -33,7 +33,7 @@ Harness spec 1.1.0: planning as one structure shared with LLeMbas.
 - `_lembas/session/rewind` (capability `rewind`): the web UI editing or regenerating from a turn
   takes that turn and every later one out of the conversation and the history — as `/undo`
   does, through the same store marking — without touching the files; a session a terminal runs
-  is rewound there, and its transcript redrawn.
+  is rewound there, and its transcript redrawn. A rewound turn's id is forgotten, so the web UI re-sending it runs the turn anew rather than being answered what the reverted turn said.
 - Prototypes in the terminal: a completed `prototype-chat` / `prototype-page` fence in a reply
   is saved as a file — the model's output as it is, under
   `~/.local/state/lembas/prototypes/<session>/<n>-<slug>.html`, never in the project — and drawn