Dump the entire KG for visualization

Single-round-trip dump of the KG suitable for d3 / cytoscape / reactflow. Each edge references its endpoints by node public_id, so the FE resolves them against nodes in the same payload — no follow-up calls.

The frontend defaults to the Tier-3 domain view (filter nodes to tier == 'domain' and the edges between them); representation nodes and their represents / cross_references edges are the drill-down evidence layer (see kg-redesign-DESIGN.md §UI mapping).