[mkotelnikov]

Stacked on #14: the base branch is #14's, so the diff shows only this change. Merge #14 first.

Problem

On workerd and celld, a promise the Worker leaves rejected is the Worker's affair: workerd logs it, celld ignores it. On Node, the Worker shares the host's process, so such a rejection reached the process's unhandledRejection listeners.

workerd's vendored Durable Object tests leave four of them (assert.rejects(…) unawaited). vitest counted them as errors, so vitest run in packages/cellb exited 1 with every test passing, and the root pnpm test stopped there.

Change

• src/core/worker-rejections.ts (new, Node only): every Worker event runs in an AsyncLocalStorage scope naming its runtime. Node calls unhandledRejection listeners in the rejected promise's async context, so a hook on process.emit finds the scope and passes the rejection to the runtime's onUnhandledRejection instead of the process. Rejections outside any Worker reach the process as before. Browsers are unaffected: a Worker runs in its own worker there, and the scope is a no-op.
• Events in scope:
  • Durable Object events, through the runtime's events root;
  • the stateless fetch;
  • code the host dispatches itself, through the new runtime.runEvent(fn), which cron triggers now use;
  • queue batches, which arrive as Durable Object events.
• CellRuntimeOptions.onUnhandledRejection: the default writes one line on stderr, [cellb] <script>: unhandled rejection in Worker code: …, outside the Worker's console, so the user logs compared with celld's recordings do not change.
• Docs: docs/celld-compatibility.md describes the behaviour; the root README's note now explains the stderr lines instead of the exit-1.
• Also merged in: Examples move to examples/; the root README gets the comparison with celld #14's fake-openai move (packages/ → examples/), on Examples move to examples/; the root README gets the comparison with celld #14's branch.

Tests

• test/p20-worker-rejections.test.ts: rejections from a fetch (before and after an await), a Durable Object method and runEvent reach onUnhandledRejection only; the host's own rejections still reach the process; the default writes the stderr line and nothing to console.error.

• p09-celld-conformance now exits 0. Its four rejections are reported under the Worker that raised them (wd-actor-stub), no longer attributed to the test vitest was running.

• Root pnpm test exits 0:

  Workspace	Result
  fake-openai	3 passed
  cellb	358 passed, 6 expected fail
  agents	2 passed
  flue	6 passed

• Interop: 123 passed, 6 expected fail; exit 0.

• Chromium: 208 passed, 1 failed, 15 skipped. The failure was tabs.spec.ts › a restarted ServiceWorker gets the shards of every tab back (Failed to fetch around the ServiceWorker restart). It passed 5 out of 5 when rerun on this branch, and this change does nothing in browsers.

• Flue in the browser: 8 passed.

• Lint and typecheck are clean in all 4 workspaces.

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