Implement forest-first layout and tuning controls

[StoneHub]

Summary

• add Forest layout mode, default to it in development, and keep the Web mode toggleable for fallback
• extend graph state/metadata and GraphManager to support forest hierarchy, pinned roots, and calmer rendering
• refresh visuals, performance, and controls (layout selection, forest tuning, optimized background) so dense trees stay responsive

Testing

• Not run (not requested)