Design Studio: durable exploration, verified prototypes, and reviewed handoff

[sambitcreate]

Design now supports durable, screen-centered projects through Explore, exact-revision Refine, project Design Language, verified prototypes, and reviewed export or workspace handoff. This PR includes the Design workspace foundation and integrates current main.

• Explore 2–4 alternatives, preserve partial results, choose a direction, and refine a specific immutable Screen revision.
• Review and apply local Design Language snapshots with exact hashes and workspace freshness checks.
• Save bounded prototype links and an explicit start Screen. Trusted Electron input must exercise the sandbox bridge before verification is recorded; changed sources invalidate playback.
• Export a deterministic reviewed project ZIP with exact Screen sources, PROJECT.md, DESIGN.md, optional prototype graph, and selected references. V2 handoff journals preserve V1 recovery and revalidate the full reviewed scope before effects and model use.
• Preserve Design ownership through startup cleanup, duplication, recovery, and native/remote exclusion. Keep export review state usable after Refine, Detach, and Connect App.

Validation: two independent GPT-6 Astra medium reviews after each phase and a combined review, with every reported finding fixed and re-reviewed. Full npm regression passed, including the 1,598-test final suite. Type checks, lint, production build, onboarding, Design recovery/policy, 10 browser scenarios, and two Electron Design scenarios passed. Android unit/test compilation and generic iOS hardware test compilation passed during integration. The Developer ID signed development package passed hardened verification and isolated V2/language create, apply, restart, and detach acceptance. Hosted CI is attached to the exact PR head. Notarized distribution, VoiceOver, physical-device, and credentialed live-provider acceptance remain separate operator gates.