Service level objective

[gloskull]

Motivation

• Provide a shared, deterministic SLO evaluation library so all VeriNode services and off‑chain consumers use the same rules for 99.99% availability and <100 ms P99 critical‑path latency.
• Surface compact SLO events for monitoring, alerting, canary analysis and runbook automation so alerts and dashboards remain consistent across services.

Description

• Add src/slo.rs that defines SLO constants, SloTarget, SloWindow, SloEvaluation, and the evaluate_window and publish_slo_evaluation helpers.
• Implement burn‑rate math (observed_error_bps, burn_rate_x100) and latency violation logic, and encode alert signals as SloSignal::{Healthy,Ticket,Page}.
• Export the module from the crate by adding pub mod slo; to src/lib.rs and include a human‑facing SLO_MONITORING_RUNBOOK.md describing architecture, alert policy, dashboards, canary checks, and incident response.
• Add unit tests in src/slo.rs covering default targets, healthy state, ticket threshold, page threshold, and latency‑based paging.

Testing

• Ran the unit tests for the SLO module with cargo test slo --lib, which executed the SLO test suite and reported 5 passed; 0 failed.

Closes #89