Add Operation::Page: a PageIntent records the page model the editor started
from, the model it produced and the author. Replica::save publishes the
difference locally through PreparedEdit::page and, while the newest pending
edit is an unattempted save of the same page, replaces that save instead of
queueing another publication, matching OneNote's own save interval
(evidence/m10/NATIVE-SAVE-CADENCE.md). Reconciliation compares the remote
page with the recorded model: an unchanged page publishes, a changed one is a
ContentChanged conflict that review_page resolves with a model reviewed
against the remote image. The page model and the document types it embeds
gain Clone and serde support; image payloads are identified by their id and
left out of equality and serialization.

Validation: six new sync tests (publication, coalescing, immutability of an
attempted save, conflict and review, independence between pages, reopen);
329 notebook and onestore tests, Clippy and fmt.

Assisted-by: claude-fable-5.1