Real-time progress streaming

[JSONbored]

Problem: A customer has no way to watch their own loop's progress other than polling for a final result.

Area: Product / API

Proposal: Stream loop progress (current step, status, recent activity) to the customer in near-real time.

Deliverables:

• A progress-streaming mechanism a customer-facing surface (Customer-facing dashboard #4807) can consume directly.

Acceptance criteria:

• A test loop's progress is visible, updating, without the client needing to poll on a fixed interval.

Boundaries:

• Blocked on Task-graph → claim → code → submit wiring #4799.

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Part of #4778.

[linear-code]

JSO-3472