feat(react-tree-grid): add threaded virtualization story

[bsunderhus]

Summary

• add a new ThreadedVirtualization TreeGrid story that adapts a threaded navigation reference into a flattened virtualized TreeGrid
• keep thread semantics through TreeGridRow structure, open state, and row typing instead of treating the experience as a generic grouped list
• demonstrate virtualization-aware focus recovery with a story-owned focus registry and scrollToItem coordination
• wire the new story into Storybook exports

How this differs from the source

• the source pattern is a bespoke threaded experience, while this version intentionally exercises TreeGrid primitives under virtualization
• this version uses header, message, and reply row kinds to preserve thread structure inside a flattened visible-items model
• this version still carries a small set of navigation overrides where current TreeGrid and Tabster behavior cannot yet express the intended thread traversal directly

Current direction

• we are still polishing the example with the goal of reducing story-level navigation overrides so more of the final behavior can come from the platform primitives themselves

Validation

• yarn nx run react-tree-grid:lint

Notes

• Beachball change files are not required here because *.stories.tsx and *.md are ignored by the repository Beachball config.