Slate v2 ReactEditor DOM Helper Recovery

Goal

Close the next honest ReactEditor / slate-dom helper gap in one coherent batch.

Current Batch

1. split clipboard fragment-vs-text insertion on the current bridge
2. recover DOM target classification helpers
3. recover findEventRange over the mounted root and current point/range seam
4. widen runtime proof and sync docs/ledgers

Notes

• do not recreate the old DOMEditor stack wholesale
• recover only the helpers the current mounted bridge can prove
• keep docs aligned to the live helper surface instead of legacy overclaim

Result

• split the clipboard bridge into insertFragmentData, insertTextData, and the generic insertData path
• restored current DOM target classification helpers on ReactEditor: hasTarget, hasEditableTarget, hasSelectableTarget, and isTargetInsideNonReadonlyVoid
• restored ReactEditor.findEventRange over the mounted root caret APIs and the current void-target seam
• widened runtime proof for: custom clipboard format keys, split clipboard insertion, mounted target checks, and event-range resolution
• updated the ReactEditor docs and the RC ledgers so the recovered helper seam is explicit

Verification

• yarn workspace slate-react run test -- --test-name-pattern "withReact and ReactEditor expose|DOM target and event helpers expose"
• yarn workspace slate-react run test
• yarn test:custom
• yarn lint:typescript