feat(studio): focus pair diagnostics in viewport

[w1ne]

Summary

• add pair-aware viewport focus metadata while preserving single primary selection targets
• focus Validity row/card actions on both parts of pair diagnostics
• fit the Studio camera to focused assembly part geometries

Verification

• npm test -- --run src/studio/tests/diagnosticRouter.test.ts src/studio/tests/adapters/validitySuggestions.test.ts src/studio/tests/shellStore.test.ts src/studio/tests/tabs/ValidityTab.test.tsx src/studio/tests/viewportFocusTarget.test.ts
• npm run typecheck
• npm run lint
• npm run build:studio