Stage generated Studio edits safely

[w1ne]

Summary

• Route Studio Generate results into the staged edit review slot instead of directly applying code.
• Snapshot prompt, target, repair workflow, and exact baseline at generation start so stale edits cannot overwrite newer editor changes.
• Block stale staged-edit approvals and distinguish staged vs discarded generated proposals.

Test Plan

• npm test -- --run src/studio/tests/StagedEditSlot.test.tsx src/studio/tests/StudioGenerate.test.tsx
• npm run typecheck
• npm run build:studio
• npm run lint