Add drush myrest:scan with readable YAML output

Problem/Motivation

The map is the thing a human has to read before trusting anything generated from it. It also has to be readable in a deployment where nobody opens an administration page.

Proposed resolution

• drush myrest:scan writes the map as YAML, to a file or to stdout, with a stable key order so two runs diff cleanly.
• Options to limit the scan to one entity type or bundle, because on a large site the full map is long.
• A summary line per bundle — how many fields, how many have a shape, how many do not — so the useful number is visible without reading the whole file.
• Exit codes that mean something for CI.

Remaining tasks

Everything.