docs(readme): Requirements says "Node.js 18+" but engines is ">=20.0.0" and CI only tests 20+22

[CryptoJones]

Problem

README.md § Requirements line 94:

- **Node.js 18+** (tested on 20 and 22)

But:

• package.json declares "engines": { "node": ">=20.0.0" }
• The README's own badge already says Node.js-20%2B
• The CI matrix only tests Node 20 and 22

A contributor on Node 18 trusting the Requirements line would
npm install and get an engines-mismatch warning that contradicts
the line they just followed. Pure doc rot — 18+ from before the
engines pin got bumped.

Fix

Change Node.js 18+ to Node.js 20+. One-line change.

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