Slate v2 History Helper Recovery

Goal

Recover the remaining cheap slate-history helper surface that the current docs still claim.

Completed

• restored HistoryEditor.withNewBatch(...)
• restored HistoryEditor.withoutMerging(...)
• restored the editor.writeHistory(...) instance seam in the public interface
• routed commit-time undo/redo stack writes through editor.writeHistory(...)
• mapped split-once behavior onto the current commit-subscriber history writer
• widened history-contract.ts to prove split-once batching, hard non-merge, and the writer seam

Verification

• yarn test:custom
• yarn lint:typescript