[AbirAbbas]

Summary

An engine that serves collections now also announces collections.factory_job in hello.features.

This wire already validates factory_job refs (a Factory job by factory and job number) and passes them to every collector. An older engine that announces only collections may refuse the kind with bad_params, and a client had no way to tell the two apart before it tried. With the flag, a client can check before it adds a job to a collection, instead of creating a Factory job and only then finding that it cannot be grouped.

Generic and additive: one new file with the constant and its hello helper, one registration line in the hello assembly, a PROTOCOL.md sentence. No change to any method, ref shape or store.

Validation contract

• An engine without collections never announces collections.factory_job: TestCollectionsFactoryJobFeatureFollowsCollections, and the plain-engine half of TestCollectionsFeatureFollowsTheEngine.
• An engine with collections announces both collections and collections.factory_job: TestCollectionsFactoryJobFeatureFollowsCollections, TestCollectionsFeatureFollowsTheEngine (its exact feature list gains the new flag).
• The announced feature is served: a factory_job ref with its factory reaches the collector, and one without a factory is refused with bad_params first: TestCollectionsFactoryJobFeatureIsServed.

A client can gate Factory-job grouping on this flag, so on an engine without it no job is created that cannot then be grouped.

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