* claim: #3242
* MCP batch: rollback restores the design the batch began in (#3242)
A failed batch with rollback: true reported rolled_back: true even when the
design never went back. Two defects, both fixed:
- file.new (and any document-replacing command) replaced the session with it,
taking its checkpoints down too, so a batch that failed after file.new had
nothing to restore. Session::replace_design now carries the checkpoints over
to the fresh session: the checkpoint belongs to the batch, not to the
session that happened to be holding the document.
- rolled_back echoed the requested flag. It now reports whether the design is
actually back: the app answers engine.batch with a new restored field, and
the MCP tool reports restored && rollback. An app that predates the field
falls back to undone/completed.
* Batch: answer with main's rolled_back field after the merge (#3242)
Upstream #2900 put the real restore check on engine.batch as
`rolled_back` while this branch named it `restored`; the merge kept
main's control test and this branch's field, and CI caught it.
Standardize on main's wire name: the app answers `rolled_back`, the
MCP tool reads it (fallback for older apps unchanged), docs and tests
follow. The checkpoint carry-over across file.new is unchanged.
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