Pluggable observability for Cognis.

Bridges [cognis_core::CallbackHandler] events to external observability backends. Phase 1 ships the bridge handler, three core types, two built-in exporters (stdout + mock), and the Langfuse backend (traces, prompts, scores). LangSmith and OpenTelemetry are Phase 2.

Quick start

use cognis_trace::{MockExporter, TraceMeta, TracingHandler};
use cognis_trace::meta::merge_into;

# async fn main_inner() {
let handler = TracingHandler::builder()
    .with_exporter(MockExporter::new())
    .with_default_pricing()
    .build();

let metadata = merge_into(serde_json::Value::Null, TraceMeta::session("session-abc"));
# }

See docs/superpowers/specs/2026-05-06-cognis-trace-design.md for the full design.