Visual line navigation helpers — implements Word/Google-Docs-style ArrowUp / ArrowDown with sticky X across visual lines (not just paragraphs). Lifted from packages/react/src/paged-editor/ useVisualLineNavigation.ts so both adapters share the algorithm.

Frontend-agnostic: takes a `getContainer: () => HTMLElement | null` callback and a mutable sticky-state object, returns the same function quartet React's hook returns.

Tagged `@internal` post-1.0 cut. Both adapters re-export this through their own composables (`useVisualLineNavigation`); consumers should prefer those. The subpath stays in `package.json` `exports` for back-compat; expect it to move behind a public surface in a future major.