feat(studio): group duplicate validity diagnostics

[w1ne]

Summary

• group duplicate diagnostic suggestion cards by diagnostic code plus routed target identity
• keep raw diagnostic rows complete while showing a compact findings count on grouped cards
• apply the visible suggestion limit after grouping so duplicates do not crowd out other failures

Verification

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