Add Next item translation sidecar

[hta218]

Summary

• add Next item-level translation sidecar types and exports
• overlay item translation values in WeaverseNextItem.getSnapShot() without mutating base _store
• implement runtime sidecar methods: extractTranslationSidecar, setTranslationSidecar, updateTranslation, getTranslationChanges, setProjectData
• cover non-design runtime reuse and design-mode live sidecar preservation

Verification

• pnpm --filter @weaverse/next test -- __tests__/next-adapter.test.tsx — 5 files, 118 tests passed
• pnpm --filter @weaverse/next typecheck — passed
• pnpm --filter @weaverse/next build — passed
• pnpm exec biome check packages/next/src packages/next/__tests__ packages/next/README.md --diagnostic-level=error — passed
• git diff --check — passed
• autoreview-copilot — clean