Add OTel file exporter for offline trace debugging

Goal

Provide an easy way to capture OpenTelemetry traces/spans to a file for offline debugging and analysis, without needing to run a collector or backend.

Use Cases

1. Debugging Effect programs - Capture the execution trace of an Effect across IO boundaries
2. Post-mortem analysis - Save traces to a file for later inspection
3. Development - Quick trace capture without infrastructure setup
4. Testing - Assert on trace structure in tests

Proposed API

import { OtelFileExporter } from '@overeng/utils/node'

// Option 1: Layer that writes spans to a JSON Lines file
const TracingLayer = OtelFileExporter.layer({ path: '/tmp/traces.jsonl' })

// Option 2: Effect that returns trace data
const trace = yield* OtelFileExporter.captureTrace(myEffect)
// trace: { spans: Span[], duration: number, ... }

// Option 3: Integration with @effect/opentelemetry
import { NodeSdk } from '@effect/opentelemetry'
const sdk = NodeSdk.layer(() => ({
  spanProcessor: new SimpleSpanProcessor(OtelFileExporter.make('/tmp/traces.jsonl'))
}))

Output Format

JSON Lines format (one span per line) for easy streaming and parsing:

{"traceId":"abc123","spanId":"def456","name":"myOperation","startTime":1234567890,"endTime":1234567891,"attributes":{"key":"value"},"events":[],"links":[]}
{"traceId":"abc123","spanId":"ghi789","parentSpanId":"def456","name":"childOperation",...}

Related Issues

• Effect-TS/effect#2212 - Use span attributes to enrich stack traces
• Effect-TS/effect#2815 - Add Cause.spans (resolved)

Implementation Notes

• Could build on @effect/opentelemetry or be standalone
• Consider compatibility with standard OTLP JSON format
• Should handle high-volume spans efficiently (buffering, async writes)
• Consider log rotation for long-running processes

Start by reviewing the proposed @overengineeringstudio/effect-utils/node entry point and the @effect/opentelemetry NodeSdk integration. Resolve which OtelFileExporter API and OTLP-compatible JSON Lines behavior are in scope, then verify that captured spans support the listed debugging and testing use cases.