TableSelectionManager

Framework-agnostic class for managing table cell selection state. Extracted from the React `useTableSelection` hook.

Handles: - Cell selection via data-attribute queries on the DOM - Table document operations (add/delete rows/columns, merge/split)

Tagged `@internal` post-1.0 cut. Adapters bind to this class today, but consumers should reach for `useTableSelection()` (React/Vue) instead of this raw subpath. The subpath stays in `package.json` `exports` for back-compat; expect it to move behind a public surface in a future major.