Add navigate back with result for UI entries

[kikin81]

Goal

Allow a screen to go back and return a typed result to the previous screen. Support this for UiEntry and UIExternalEntry so callers can launch an entry, await a result, and handle it in a safe way.

Scope

• Design a simple API to request an entry and await a result
• Implement result passing for UiEntry
  • Store the pending request and deliver the result on back navigation
  • Ensure single delivery, even across configuration changes
• Implement result passing for UIExternalEntry
  • Bridge external flows so they can return a result when finishing
• Handle edge cases
  • Previous screen destroyed and recreated
  • Multiple outstanding requests
  • Type safety and key collisions
• Add sample usage in sample app
  • One screen launches another, gets a result, updates UI
• Docs
  • KDoc on public API
  • README section with short examples

Outcome

• Callers can open a UiEntry or UIExternalEntry and receive a typed result when the user goes back
• Results are delivered once, survive rotation, and handle cancel paths
• Demo and tests prove the flow
• Documentation explains how to adopt it