XY-1020 Add graph topic-map reports

[yvette-carlisle]

Summary

• add elf.graph_report/v1 graph-lite topic-map reports from existing Postgres graph facts
• expose graph reports through ElfService, /v2/graph/report, and MCP elf_graph_report
• document the graph report contract and evidence boundaries without claiming external graph/RAG parity

Validation

• cargo test -p elf-service graph_report -- --nocapture
• cargo test -p elf-mcp registers_all_tools -- --nocapture
• cargo test -p elf-eval --test real_world_job_benchmark graph_topic_map_report_wires_source_backed_graph_lite_readback -- --nocapture
• cargo make lint-fix
• cargo make check-docs
• decodex docs check
• git diff --check
• cargo make check