bench: add normalized native BFS correctness gate - #18

Summary

Starts the next Issue #10 increment by adding a strict cross-engine BFS normalization gate.

• requires identical dataset SHA-256, source, directedness, vertex/edge counts
• requires identical reachable-vertex counts
• requires identical full BFS distance-vector digests across builtin, LAGraph and GAP
• keeps hosted results research_claim: false, publication_grade: false, and publication_ready: false
• documents the native runner contract and remaining implementation needed to expose full distance vectors from pinned LAGraph/GraphBLAS and GAP

This PR is intentionally a draft until the pinned native runners are wired into hosted CI and the normalized gate passes end-to-end.

Refs #10