feat: PulseStreamHub and MapStreamQueryHub() for SignalR-based stream queries (Low Priority)

[samtrion]

User Story

As a developer building a real-time UI, I want to subscribe to IMediator.StreamQueryAsync results via a SignalR hub, so that I can push live data to browser clients without polling.

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Background

⚠️ Low priority — implement only after all Tier 1–5 items are complete.

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Requirements

• Create src/NetEvolve.Pulse.AspNetCore/Hubs/PulseStreamHub.cs:
  • Hub subclass with StreamAsync<TQuery, TResponse>(TQuery query).
  • Iterates IMediator.StreamQueryAsync<TQuery, TResponse>(query, cancellationToken) and sends each item via Clients.Caller.SendAsync("OnItem", item).
  • Respects client-side cancellation.
• Add MapStreamQueryHub<TQuery, TResponse>(this IEndpointRouteBuilder, string path) extension method.
• Unit tests for hub and extension method.

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Acceptance Criteria

• Hub iterates the stream and sends all items to the caller.
• Client cancellation stops the stream without exceptions.
• MapStreamQueryHub() registers the hub at the specified path.
• Unit tests pass.