Fix embedded Fabric custom view sizing (0.7.3-patch2)

[asikora-sf]

Summary

• Measure the mounted Fabric root component when its proxy surface remains at the 1-point bootstrap height, so embedded custom views can expand in Agentforce conversations.
• Keep observing while mounted and update the SwiftUI host when React Native content grows or shrinks.
• Bump the bridge package to 0.7.3-patch2.

Validation

• Focused iOS simulator tests for a 1-point host with 160-point content, subsequent size changes, and pre-mount state.
• Swift source parse, bridge TypeScript build, and npm pack dry run. The packed package includes the new Swift helper.