Split `EditorStore` into smaller services

[kulla]

Problem: EditorStore manages CRDT data, schema lookup, transactions, rich-text editors, selection, and update subscriptions.

Improve: split responsibilities.

class DocumentRepository {
  getNode(key: Key): FlatNode
  saveNode(node: FlatNode): void
}

class TransactionManager {
  update(fn: (tx: Transaction) => void): void
}

class RichTextManager {
  getEditor(key: Key, schema: RichTextSchema): Editor
  destroyEditor(key: Key): void
}

class SelectionStore {
  getSelection(): EditorSelection | null
  setSelection(selection: EditorSelection | null): void
}

class EditorRuntime {
  constructor(
    readonly document: DocumentRepository,
    readonly transactions: TransactionManager,
    readonly richText: RichTextManager,
    readonly selection: SelectionStore,
  ) {}
}

This improves testing and makes each layer simpler.