Pin production nan to an exact version #6

package.json ships "nan": "^2.23.1". NAN is compiled into the addon (binding.gyp require('nan')). A caret is not a pin: every consumer npm install re-resolves >=2.23.1 <3.0.0. This repo's lockfile hashing 2.28.0 does not bind dependents.

Pin nan to the reviewed exact version (currently 2.28.0). Prefer a shrinkwrap (or equivalent dependents actually apply) with integrity hashes. CI should install from the lockfile (npm ci), not a floating npm install.