graph_accel: CI pipeline, standalone repo, and PGXN submission

[aaronsb]

Context

Phase 6 of ADR-201 — publishing graph_accel as an independent open-source PostgreSQL extension.

Items

• Set up CI: cargo pgrx test + standalone benchmark regression
• Publish to GitHub as standalone repo (Apache 2.0)
• Submit to PGXN (PostgreSQL Extension Network)

Already done

• graph_accel_degree() — degree centrality
• graph_accel_subgraph() — subgraph extraction
• Full benchmark suite and documentation