feat: add ElevenLabsWidget with a runnable DemoApp

[renal128]

Summary

First slice of the widget port. Adds ElevenLabsWidget as a second product in the package and an Examples/DemoApp that uses it, so everything after this is reviewable against something you can actually run.

• Widget: floating launcher, drawer, transcript, composer, mic mute, a static placeholder orb.
• ChatWidgetController: the host's handle on the widget — mirrors state (state, isOpen, isMicMuted, conversationId, messageCount) and forwards commands (open, close, startConversation, sendMessage, setMicMuted, ...).
• ChatWidgetConfig / ChatWidgetStrings / ChatWidgetTheme for behavior, copy, and colors.
• Examples/DemoApp: a real-estate search form driven by an update_real_estate_search client tool, plus buttons that drive the widget through the controller.

Everything is #if canImport(UIKit) and @available(iOS 16, ...), so the SDK keeps its iOS 13 floor and swift build on macOS compiles the widget to an empty module.

Not in this PR

The real orb (next), conversation modes, drawer polish, markdown transcript, error banner, attachments, feedback, terms, and MCP approval all land as their own slices on top.

Test plan

• swift build, swift test, swiftformat . --strict
• xcodebuild of Examples/DemoApp for the iOS simulator
• Manual: launcher opens the drawer, text conversation round-trips, client tool fills the form

Note that repo CI only builds PRs targeting main, so this stack gets SwiftFormat but no build job.

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