Add Crosswalk Visualizer page — interactive graph of risks and frameworks

Flat D3 force-directed graph with category filter chips, search,
side panel detail view, and click-through to Explorer. Risk nodes are
sized uniformly; framework nodes are sized by control count and edges
are weighted by mapping count.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: emmanuelgjr