Slate v2 Slate-react Surface Recovery

Goal

Close the next honest packages/slate-react/src public-surface gap in one batch.

audit the remaining missing slate-react public names against the current runtime shape
recover the honest hook/context/default-alias surface
recover a current ReactEditor / withReact seam only where the bridge can actually prove it
widen runtime proof and sync the proof ledgers

do not fake the old slate-dom plugin stack
restore only the names that can be backed by the current React + DOM bridge
if a legacy surface is still too broad, cut the overclaim in docs instead of lying in code

restored useElement, useElementIf, and useSelected on the current render-element seam
restored default component aliases: DefaultElement, DefaultLeaf, DefaultText, and DefaultPlaceholder
restored withReact as a compatibility construction helper that records the current clipboard fragment format key without wrapping the editor instance
restored a current ReactEditor namespace over the mounted bridge: isComposing, isFocused, isReadOnly, blur, focus, deselect, findKey, findPath, hasDOMNode, toDOMNode, toDOMPoint, toDOMRange, toSlateNode, toSlatePoint, toSlateRange, insertData, and setFragmentData
widened the mounted DOM bridge enough to support that helper seam honestly
updated the slate-react docs to describe the current proved helper surface instead of the old plugin-era overclaim
updated the True Slate RC proof ledger and file-review ledger to carry the recovered slate-react surface explicitly