heliosdb-nano-observability

Tracing, slow-query logging, health checks, statistics, and metrics in HeliosDB-Nano. Covers RUST_LOG filters, the WARN-level slow-query log (1 s default threshold), the GET /health HTTP endpoint, REPL diagnostic commands (\stats, \compression, \optimize, \indexes), and Prometheus integration where wired. Use this when the user is debugging performance, asking "why is this slow", monitoring uptime, or wiring Nano…