[P3] Declare Node 24 in machine-readable project configuration #356

Priority

P3 - local environments can drift from CI and fail with unclear toolchain errors.

Finding

README, CONTRIBUTING, and all workflows use Node 24, but package.json has no engines or devEngines field and the repository has no .nvmrc, .node-version, Volta, or asdf declaration.

Acceptance criteria

• Add a package.json engine declaration and a version-manager file aligned with CI.
• Keep the documentation and performance reference synchronized.
• Verify npm ci and validation on Node 24 and provide a clear failure on unsupported versions.