feat(effect-sdk): opt-in native Cloudflare tracer for the Workers preset

[Makisuo]

What

Adds an experimental tracer: "native" mode to @maple-dev/effect-sdk/cloudflare. Every sampled Effect span is mirrored onto tracing.startActiveSpan from cloudflare:workers, so it joins the same trace as Cloudflare's auto-instrumented fetch/KV/R2/D1 spans and is exported by the customer's ObservabilityDestination. No in-isolate buffer, no ctx.waitUntil(flush), no ingest key in the Worker. The default OTLP mode and the api/alerting workers are unchanged.

Why

The OTLP buffer flush is unreliable from Durable Object and Workflow isolates and requires an ingest key in the Worker. Cloudflare's own tracing (compatibility date >= 2026-07-28) can carry the spans out instead.

How

• cloudflare/native-tracer.ts: an Effect Tracer.make over startActiveSpan. Each mirrored span captures AsyncLocalStorage.snapshot() inside its callback, children open inside the parent's snapshot, and the context hook runs every fiber step inside the current span's snapshot. Cloudflare's isTraced cascades into Effect's sampled, so an untraced invocation opens no Cloudflare spans at all.
• Failures become attributes (exception.type, exception.message, exception.stacktrace, error.type) since Cloudflare spans have no events. A parallel change teaches the error MV to read those attributes.
• Scalar attributes are forwarded on set; objects, arrays, bigints, events and links stay Effect-local.
• shared/span-exit.ts: the success / interrupted / ignored / anticipated / failed classification is shared between the OTLP buffer tracer and the native tracer so they cannot disagree. The OTLP tracer's behaviour is unchanged.
• cloudflare:workers and node:async_hooks are imported dynamically by a variable specifier and narrowed by type guards (the installed workers-types only know enterSpan). A missing API fails with NativeTracingUnavailable, which the layer turns into one Effect.logInfo notice plus Effect-local spans.
• Logs are left to Workers Logs in native mode: OTLP log records would carry Effect trace ids that never match the Cloudflare trace ids on the exported spans, and would need exactly the flush and key this mode removes. Metrics are likewise not exported. Documented in the README.

Reviewer notes

• Known limitation, documented in the README: the layer is module-scoped, so it cannot capture a per-invocation snapshot. Root spans use the ambient async context at open time, and within one continuation the ambient context can still be the span that just ended. This should be validated on a real Worker before the mode leaves "experimental", along with the attribute size limit for the mirrored stack trace.
• The Config.tracer option and the README section are the only public surface changes.

Testing

• native-tracer.test.ts: a fake tracing backed by a real AsyncLocalStorage asserts parentage across a scheduler hop into a foreign async context, interleaved fibers, attribute forwarding, all five exit classes, the isTraced cascade, dropped names, host resolution failures, and the fallback path.
• index.test.ts: native mode off Workers builds the layer, makes no fetch calls, resolves flush, and logs one notice.
• effect-sdk vitest suite: 17 files, 143 tests pass. tsc --noEmit, oxlint and oxfmt clean on the package. tsdown build confirmed the dynamic imports survive unbundled.

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