Slate v2 Public/Core Diff Closure

Goal

Close the next big packages/slate/src public/core gap with one coherent batch, not another one-method-at-a-time crawl.

Current Batch

1. audit legacy packages/slate/src editor/static helper files against the current public surface
2. recover a coherent static read/query helper family in Editor.*
3. prove the family in snapshot-contract.ts
4. update file-granular ledger rows so the remaining holes are explicit

Notes

• the obvious open hole is the static read/query layer, not another transform seam
• restore helpers that fit the current data-model-first engine
• do not fake iterator-width or legacy breadth that the current engine still cannot prove

Result

• restored a coherent Editor.* static read/query batch for the current live-tree seam: edges, first, start, end, last, parent, path, point, range, node, string, fragment, hasPath, hasBlocks, hasInlines, hasTexts, isBlock, isEmpty, isStart, isEnd, and isEdge
• widened that same family onto the editor instance surface and added the sibling/iterator helpers next, previous, levels, nodes, leaf, and void
• restored the current headless ref seam: pathRef, pathRefs, pointRef, pointRefs, and rangeRefs
• restored the next traversal/control helpers: above, positions, normalize, isNormalizing, and unhangRange
• restored the remaining obvious docs-backed query hooks: elementReadOnly, isElementReadOnly, and isSelectable
• restored singular insertNode on both Editor.* and editor.* over the current insertNodes seam
• widened proof depth for the recovered helpers with representative legacy rows for above, positions, and unhangRange
• restored the remaining editor-instance compatibility hooks: getChildren, setChildren, getDirtyPaths, setNormalizing, shouldMergeNodesRemovePrevNode, and the enumerable children accessor
• widened the exported utility surface for: Path.*, Point.*, and Range.*
• proved those helpers against both committed snapshots and live transactions in snapshot-contract.ts
• closed file-granular review for the current packages/slate/src tree in release-file-review-ledger.md
• added a matching public-surface proof row to true-slate-rc-proof-ledger.md
• updated the API reference front door so the recovered static surface is explicit and the still-missing iterator/query breadth is called out
• closed the already-green proof rows for range-ref-contract.ts, clipboard-contract.ts, and history-contract.ts so those lanes are no longer marked pending out of laziness

Verification

• yarn test:custom
• yarn lint:typescript