Build a Loopback-Only Local API

Mission

Create a small validated service that listens only on localhost and never exposes debug mode to Wi-Fi.

Required work

1. Bind explicitly to 127.0.0.1
2. Validate input length and type
3. Return consistent JSON errors
4. Confirm no LAN listening socket exists

Evidence to produce

A local API, request tests and a network-exposure verification note.

Quality checks

The result must name its sources, separate observation from assumption, preserve originals and explain limitations.