Add a consumer example beyond Google Apps Script

[headerhound-dev]

loader.gs is the reference consumer for Gmail. HeaderHound is meant to be consumed by any mail filter. It'd be great to have small examples for other environments — a Sieve script, a Fastmail rule, an mailscript/Cloudflare Email Worker, or a simple Python/Node snippet that reads fingerprints.json and tests a .eml file.

Pick one environment and add a short, self-contained example under a new examples/ folder.