effect-v4-observability

Use when adding logging, metrics, tracing/spans, or OpenTelemetry to Effect v4 code — covers Effect.fn named spans, the Effect.log* family, custom loggers (Logger.make for the format, Logger.withConsoleLog/withConsoleError for the route — never a hand-written Console.log call), Metric counters/gauges/histograms with Metric.withAttributes (NOT the removed Metric.tagged), and wiring @effect/opentelemetry at the app edge. Encodes the house rule that pure-tier libraries instrument public fallible boundaries only and stay telemetry-agnostic, while apps compose OTel at the infrastructure layer.