Add .nvmrc file to enforce Node environment constraints

[Code-Paragon]

Problem

The README.md specifies Node.js v18+, but there is no mechanism to enforce or auto-switch to this version for contributors cloning the repo.

Expected behavior

An .nvmrc file is placed in the root directory specifying v18.

Files to update

• .nvmrc (create)

Project relevance

Prevents obscure dependency build failures caused by users accidentally running older Node.js runtimes locally.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• File contains exactly v18 or 18
• No TypeScript, lint, test, or build errors