docs(mailcore): add README explaining license and module layout

mailcore/README.md

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+# mailcore
+
+The open-source Rust engine at the heart of [Korax](../) — a PGP-first email client.
+
+This library handles everything below the UI layer: IMAP, SMTP, MIME parsing, local SQLite storage, and DNS auto-discovery. It is published here for transparency and community audit.
+
+## What's here
+
+| Module | Description |
+|---|---|
+| `src/imap/` | RFC 3501 IMAP client, UIDVALIDITY-aware, uses `BODY.PEEK[]` to preserve `\Seen` flag integrity |
+| `src/smtp/` | RFC 5321 SMTP with STARTTLS and SMTPS |
+| `src/storage/` | SQLite message cache, folder state, account credentials |
+| `src/mime/` | RFC 2045/2046 multipart MIME parser |
+| `src/dns/` | SRV auto-discovery (`_imaps._tcp`, `_submission._tcp`) — same mechanism used by Thunderbird and Evolution |
+| `src/session_manager.rs` | Shared IMAP session pool |
+| `src/api.rs` | Public FFI surface (consumed via `flutter_rust_bridge`) |
+
+## What's not here
+
+PGP/Sequoia integration and advanced privacy features live in a separate proprietary crate (`mailmore`) and are not published.
+
+## License
+
+Published under the **PolyForm Noncommercial License 1.0.0** — free to read, study, and use for noncommercial purposes. See the [LICENSE](../LICENSE) file in the root of this repository. Commercial use requires a separate agreement with the author.
+
+© Eugen Kaparulin. All rights reserved.