Write down everything ihasmail does

The feature list lived in three places that each answered a different question: the site sells it, the docs teach the parts that surprise people, and the README summarises both in a paragraph. Nothing said, in one place and at full detail, what is actually built -- so evaluating ihasmail meant reading the source, and a feature that quietly stopped working had nowhere to be contradicted.

FEATURES.md is that inventory, written from the code rather than the copy: the capability matrix and what each missing one removes, the search operators as the parser actually reads them, the Sieve tests and actions, every settings section and which of them follow the account, the shortcut set, the security posture, and the environment. Where a behaviour is odd it says why, because the reason is usually the stateless constraint.

It also records what the docs had not caught up with: single-occurrence edit and delete are built, and the id renumbering that made them hard is described where someone changing that code will find it.

README.md

@@ -24,6 +24,7 @@ durable belongs to Stalwart; the container is disposable.
 | --- | --- |
 | 🌐 **[ihasmail.org](https://ihasmail.org)** | What it is, what it looks like, the full feature list |
 | 📘 **[docs.ihasmail.org](https://docs.ihasmail.org)** | [Installing](https://docs.ihasmail.org/install/) · [Configuring](https://docs.ihasmail.org/configure/) · [Using it](https://docs.ihasmail.org/using/) · [Shortcuts](https://docs.ihasmail.org/shortcuts/) · [Rebranding](https://docs.ihasmail.org/rebranding/) · [Troubleshooting](https://docs.ihasmail.org/troubleshooting/) |
+| 📋 **[FEATURES.md](FEATURES.md)** | Everything it does, feature by feature, with the capability each one needs |
 | 🧪 **[KNOWN-ISSUES.md](KNOWN-ISSUES.md)** | What was verified live, and where Stalwart departs from a spec |
 | 🛣 **[ROADMAP.md](ROADMAP.md)** | What ihasmail does not do, and why |