Add registerEvent to the Swift SDK

[V3RON]

Part of #95. Needs #124. Can run alongside #127.

Why

Swift apps post events that agents can't discover. They need a way to declare them, like registerTool.

Expected outcome

registerEvent(EventDescriptor) throws -> EventRegistration in the Swift core, shaped like registerTool. The SDK sends a snapshot of all declared events after every ack that carries event_registry: true, and a delta for each later declaration or disposal.

Constraints and non-goals

• With no event_registry flag in the ack (an older CLI), the SDK sends no event frames and the session keeps working.
• Declared names follow the posted-name rule (any string up to 4096 characters).
• No development warnings for undeclared names or failing payloads; that's React Native only.

How we know it is done

1. The Swift suite passes fixtures/event-descriptors.json.
2. Every ack carrying event_registry: true, including the ack after a resume, is followed by a snapshot of the current declarations.
3. Calling the returned registration's disposer sends a remove delta.
4. With no flag in the ack, no event frames are sent.
5. A frames fixture covers the snapshot and both delta operations, and the Swift suite passes it.

Alternatives considered

none