Slate v2 Slate Provider Recovery

Goal

Recover the honest <Slate> provider surface that the live docs and examples already depend on.

Current Batch

1. restore initialValue initialization on the current editor/snapshot model
2. restore onChange, onValueChange, and onSelectionChange
3. prove initialization and callback semantics in runtime tests
4. align docs and RC ledgers to the recovered provider contract

Result

• restored initialValue on the current provider seam, with one initialization pass per editor instance
• restored onChange, onValueChange, and onSelectionChange
• moved focused/readOnly/composing hook state to the provider seam so toolbar siblings can observe it too
• made value and selection callbacks diff snapshots directly instead of relying on raw operation classification, so replace() and similar helpers stay honest
• updated the Slate component docs to match the live provider surface
• updated the True Slate RC proof ledger and file-review ledger to carry the recovered provider contract explicitly

Verification

• yarn workspace slate-react run test
• yarn test:custom
• yarn lint:typescript