feat: precise --changed via stack-trace source file tracking; OTel sink docs

docs/observe.md

@@ -228,12 +228,73 @@ Key constraints:
 ## Sink Implementations
 
 APOPHIS does not ship with built-in sinks. The `ObserveSink` interface lets you
-plug in any backend. Common patterns:
+plug in any backend.
 
-- **OpenTelemetry**: emit counters and histograms via `@opentelemetry/api`.
-- **pino logger**: emit structured log records via `pino.info()` / `pino.warn()`.
-- **Internal metrics service**: POST events to an internal collector endpoint.
-- **In-memory ring buffer**: capture recent events for diagnostics endpoints.
+### OpenTelemetry
+
+Export contract evaluations as OTel custom metrics alongside your existing spans and traces:
+
+```typescript
+import type { ObserveSink, ObserveEvent } from '@apophis/fastify'
+import { metrics } from '@opentelemetry/api'
+
+const meter = metrics.getMeter('apophis')
+
+const contractCounter = meter.createCounter('apophis.contract.evaluations', {
+  description: 'Number of contract evaluations',
+})
+const violationCounter = meter.createCounter('apophis.contract.violations', {
+  description: 'Number of contract violations',
+})
+const durationHistogram = meter.createHistogram('apophis.contract.duration_ms', {
+  description: 'Contract evaluation duration',
+})
+
+export const otelSink: ObserveSink = {
+  emit(event: ObserveEvent) {
+    const labels = { route: event.route, formula: event.formula }
+    contractCounter.add(1, labels)
+    if (event.type === 'contract.violation') {
+      violationCounter.add(1, labels)
+    }
+    durationHistogram.record(event.durationMs, labels)
+  },
+}
+```
+
+Register it with your Fastify app:
+
+```typescript
+await app.register(apophisPlugin, {
+  observe: { enabled: true, sinks: [otelSink] },
+})
+```
+
+APOPHIS violations now appear in your OTel metrics backend (Prometheus, Datadog, New Relic, etc.) alongside latency and error data — no custom collector needed.
+
+### Console (development / debugging)
+
+```typescript
+export const consoleSink: ObserveSink = {
+  emit(event: ObserveEvent) {
+    if (event.type === 'contract.violation') {
+      console.warn('[apophis]', event.route, event.formula, event.observed)
+    }
+  },
+}
+```
+
+### pino logger
+
+```typescript
+export function createPinoSink(logger: pino.Logger): ObserveSink {
+  return {
+    emit(event: ObserveEvent) {
+      logger[event.type === 'contract.violation' ? 'warn' : 'info'](event)
+    },
+  }
+}
+```
 
 ## Monorepo Validation