desktop(architecture): define desktop target architecture and shared contract guardrails

[Whiteks1]

Summary

Define the top-level Desktop migration contract so Electron, preload, shared contracts, and the future renderer migration stay aligned.

Includes

• ADR-backed target architecture for Desktop
• shared contract boundary expectations
• migration guardrails and ordering
• explicit treatment of research_ui as transitional continuity, not target-state

Done when

• architecture direction is documented and accepted
• shared contract foundation is established for the current shell
• the next slice can move into TypeScript base real without reopening stack or boundary debates