docs: events now optional — Status headers + conditional HTML rendering

- ADR-015: Status → Optional with scaffold command + behavior note (bus absent
  means no-op, IJobQueue unaffected)
- events-and-jobs.md: prerequisite callout block explaining core-events is
  optional, how to scaffold, and which features require it
- data-flow-explainer.html: note bus is optional in BindProductionContext blurb;
  add inline comment in code snippet that bus is scaffoldable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/guides/events-and-jobs.md

@@ -2,6 +2,18 @@
 
 Walkthrough for adding cross-feature events and background jobs to a feature. For the architectural rationale, see [ADR-015](../decisions/adr-015-events-and-jobs.md) and [the full spec](../superpowers/specs/2026-05-08-events-and-jobs-design.md).
 
+> **Prerequisite — `@repo/core-events` is optional.**
+> The event bus (`IEventBus`, `InMemoryEventBus`, `PayloadJobsEventBus`) lives
+> in `@repo/core-events`, which ships as a scaffoldable package rather than a
+> permanent fixture. If `packages/core-events/` does not exist in your repo,
+> run `pnpm turbo gen core-package events` first, then wire the bus into
+> `apps/web-next/src/server/bind-production.ts` as described in the generator's
+> next-steps output.
+>
+> Background jobs (`IJobQueue`, `gen job`) work without core-events — they only
+> require `@repo/core-shared/jobs`. Cross-feature event fanout (`gen event
+> consume`) additionally requires core-events.
+
 The three rules to keep in mind:
 
 - **E0** — Events are for cross-feature decoupling. In-feature reactions are direct use-case calls.