chore: Migrate core Workflow module off of ReactiveSwift

[amorde]

Changes

Remove ReactiveSwift usage from the core Workflow and WorkflowUI modules.

The only usages there were for emitting outputs from WorkflowHosts to external consumers. The internal implementation of Workflow itself did not use ReactiveSwift at all.

WorkflowHost API changes:

- public let rendering: Property<WorkflowType.Rendering>
+ public var rendering: WorkflowType.Rendering { get }

+ public var renderingPublisher: AnyPublisher<WorkflowType.Rendering, Never>

- public var output: Signal<WorkflowType.Output, Never> { get }
+ public var outputPublisher: AnyPublisher<WorkflowType.Output, Never> { get }

WorkflowHostingController API changes:

- public var output: Signal<Output, Never> { get }
+ public var outputPublisher: AnyPublisher<Output, Never> { get }

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation

[johnnewman-square]

🤖 renderingPublisher applying dropFirst() at the public boundary removes the atomic current-value-plus-future-updates behavior previously available from Property.producer. A render between reading rendering and subscribing becomes the subject’s current value and is then dropped. Could this return renderingSubject.eraseToAnyPublisher() and move .dropFirst() to WorkflowHostingController, which already consumes the initial rendering separately?

[amorde]

This was done because previously rendering.signal produced only changed renderings so without this all existing rendering.signal callers had to include .dropFirst(), which felt like a footgun for migrations.

Because ReactiveSwift's Property provides both options, we have to pick which behavior we adopt here:

• Property.signal: Only emits updates
• Property.producer: Emits current value + updates

[amorde]

Keeping the producer semantics and letting callers use .dropFirst() feels more idiomatic (for example, matches the behavior of @Published properties), so I'll go with that.

[johnnewman-square]

Thanks Eric! Matching the idiomatic @Published behavior feels like the best approach to me, too.

[johnnewman-square]

This looks good on my end. My agent just had a few suggestions.

[johnnewman-square]

Thanks for making those tweaks.