feat: add "What's new in Node.js 26" blog article

Code-heavy tour of Node.js 26 highlights, prioritised by excitement:
Temporal API (now stable by default), Map/WeakMap upsert helpers,
Iterator.concat, crypto raw-key support and Ed25519 context, plus an
upgrade checklist for the breaking changes. Each section shows what
the new API looks like and, where useful, the older pre-26 equivalent.

https://claude.ai/code/session_01AQihcJjkxeokR9LrdeYuZY

Author: claude