Pure-reference catalog of guardrail-metric methodology for online controlled experiments. Defines guardrail metrics (metrics that must NOT degrade for an experiment to ship, even if the primary metric improves), the standard guardrail set (latency / errors / engagement / opt-out), the relationship to OEC (Overall Evaluation Criterion) per Kohavi et al., and pre-commitment of the metric set. The quantitative evaluation mechanics (per-metric alert/block thresholds, Bonferroni / Benjamini-Hochberg multiple-comparison correction) live in references/. Use when designing the metric set for a new experiment, auditing existing experiment configs, or reviewing experiment results before ship-decisions.