Design Studio: Stitch-inspired screen workflows

[sambitcreate]

Summary

• establishes the screen-centered Design Studio workbench
• adds durable Design Project V2 migration, title provenance, per-Screen presentation semantics, and main-owned mutation boundaries
• implements the phased Stitch-inspired roadmap on top of the local-first Design Workspace contract

Delivery status

• Phase 1: complete — workbench coherence, selection, navigation, inspector, responsive canvas
• Phase 2: complete — Project V2, migration, title policy, semantic ownership
• Phases 3–6: in progress on this branch

Verification so far

• npm run type-check
• npm run lint
• npm run test:generative-ui (396 service/component tests + 9 Electron containment/export tests)
• focused Phase 2 store, migration, IPC, sidebar, persistence, and Electron immediate-create tests
• two independent Phase 2 correctness/edge-case review gates