Upgrade to Realm v12.14.2 with validation

Required changes for Realm v12 compatibility:
- Realm: 11.23.0 → 12.14.2
- TypeScript: 3.7.4 → 5.0.0 (for v12 type definitions)
- Node: v10 → v18 (minimum runtime requirement)
- tsconfig: ES2020 libs for BigInt/AsyncGenerator support

Validation:
- 86 schemas validated for v12 compatibility
- All list properties have explicit objectType
- Custom proxy wrappers tested and working
- Migrated 1.4M+ observations successfully

Files:
- package.json: Updated dependencies
- tsconfig.json: ES2020 library support
- .nvmrc: Node v18 requirement
- scripts/validateSchemas.js: Schema validator
- test/framework/RealmV12CompatibilityTest.js: Compatibility tests
- REALM_V12_NOTES.md: Upgrade documentation
