[vitalyster]

Summary

First steps toward building the spectrum2 core (transport-protocol, libtransport, spectrum2, transport-plugin) without Swiften and Boost, so that spectrum2 can serve non-XMPP clients through a raw protobuf frontend. The XMPP and Slack frontends keep Swiften/Boost.

This PR covers three steps:

1. Standard library instead of Boost. filesystem, regex, function, thread, foreach, lexical_cast, Boost.Locale lowercasing (now Util::toLowerUTF8) and base64.
2. Core events are Event<Sig> callbacks instead of boost::signals2. Each event has a single listener; where a second listener existed, it chains to the first.
3. Protobuf boundary between the core and the frontends. libtransport no longer uses XMPP stanza types:
   • users are strings (the bare JID for XMPP);
   • buddies and rooms are their legacy network names;
   • statuses, vCards, participants and messages are pbnetwork types.

It also adds next/, a standalone prototype of a protobuf frontend plus an IRC backend. It is off by default (ENABLE_NEXT).

Design

• Frontend creates the User, RosterManager, UserManager and Conversation subclasses. It reports login results and carries raw XML in both directions (features.rawxml). The XMPP-specific send* and on* members are gone.
• Messages are (WrapperMessage_Type, ConversationMessage) pairs: plain message, subject, attention, delivery receipt, and the three typing states. NetworkConversation is gone; the backend listens on Conversation::onMessageToSend.
• Per-user output goes through hooks in the frontend's subclasses:
  • RosterManager: doSendBuddyStatus, doRequestAuthorization, isValidBuddyName
  • User: handleRoomJoined, sendCurrentPresence
  • UserManager: doRemoveUser
• Core User: setReadyToConnect, onStatusChanged(pbnetwork::Status), and joinRoom / leaveRoom. Joins wait until the user is connected and are redone after a reconnect.
• Core UserManager: loginUser (checks vip_only and enable_transport) and autoRegisterUser.
• XMPP frontend. These moved out of the core:
  • presence aggregation, client capabilities and the connect timeout → XMPPUser
  • presence-driven login, subscriptions and the user-removal timer → XMPPUserManager
  • MUC, carbons, message caching and OOB media wrapping → the new XMPPConversation
  • PresenceOracle, raw-XML parsing, and JID↔legacy-name mapping (new jidutils)
• Slack frontend no longer fakes XMPP presences and messages. It calls loginUser, joinRoom / leaveRoom and ConversationManager::handleMessageReceived directly.
• AdminInterface::handleQuery(string) returns the reply. The admin JID check moved into XMPPFrontend.

Behaviour changes

• Base64 decoding no longer strips trailing NUL bytes. Encoding is unchanged.
• "Ready to connect" now fires once. Before, a client with already-known capabilities could cause two Login messages to the backend.
• Raw-XML stanzas sent to the backend are copies. Before, the stored user presence got its "to" address rewritten.
• Slack connects to the legacy network as soon as the user logs in, instead of after the 5-second capabilities timeout. It also no longer posts empty messages for typing notifications.

Not done yet

• The Config parser to replace program_options.
• Asio networking. The core still uses Swift for the event loop, timers and connections.
• UserRegistry (XMPP server mode) still lives in libtransport.
• The protobuf frontend inside spectrum2.
• A CI build without Swiften.
• libtransport, the frontends and the tests stay C++17: Boost 1.78 signals2 does not compile as C++20.

Testing

• Steps 1–2: MSVC build OK, libtransport_test 155/155.
• Step 3 and next/: not compiled locally; CI is the first check.
• The unit tests are ported to the new API. Linux-only code (libpurple, the SQL backends, spectrum_manager) was reviewed by reading only.