Support bridgeless iOS custom views in 0.7.3-patch

[asikora-sf]

Summary

• Add an iOS root view factory registration hook for bridgeless React Native hosts. Custom responses and splash screens use the host app's existing RCTReactNativeFactory.rootViewFactory; apps with a valid legacy RCTBridge retain the RCTRootView path.
• Bootstrap Fabric response sizing while its surface is preparing, then update SwiftUI with the measured content height.
• Document the host startup registration and the flex: 0 root container style needed for content sized custom responses.
• Set package version to 0.7.3-patch; prerelease npm versions publish under the next dist tag.

Verification

• Built and ran an iOS simulator app with React Native 0.83.6 against the custom response service. The mapped React Native component and response properties rendered in the Agentforce conversation.
• Rebuilt the same app with the prior RCTRootView creation path. The same response was blank; its bridge proxy reported isValid = false.
• Final cleaned patch rebuilt and rendered the response again.
• Root TypeScript typecheck, bridge build, package dry run, and git diff --check passed. The dry run includes the new Swift source.

Release

Merge this PR to main before creating v0.7.3-patch. The npm publish workflow accepts only tags whose commit is on main.