Add docs/ARCHITECTURE.md: system overview, source map, threading model

[mcfnord]

Split out of #3789 per review — one new file per PR.

A single page answering the questions every new contributor (human or agent) otherwise reconstructs from scratch: how audio flows client → server → client, which class lives in which file, which threads exist and which of them are real-time, how a UDP "connection" works without a handshake, how directory servers and hole punching fit in, and where common kinds of changes go. It states two invariants: never block the real-time path, and treat all network input as untrusted.

Everything in it is verified against current main (socket.cpp ProcessPacket(), CHighPrecisionTimer/OnTimer, CChannel, CServerListManager).

Two things worth discussing here, per the comments in #3789:

• AGENTS.md stays extremely short and just links here; this file is the long-form home. If any part of this belongs in AGENTS.md instead (or vice versa), happy to move it.
• Agents can enumerate src/ themselves, so the source map is aimed at humans — it carries the class→file responsibilities that enumeration doesn't reveal. Can trim if you prefer.

Docs only; no code changes.

CHANGELOG: SKIP

🤖 Generated with Claude Code

[ann0see]

I suppose we'll need to make this more detailed. Also explain classes, ...

[mcfnord]

Added a "Key classes and how they connect" section — the ownership tree (CServer → vecChannels[] → SockBuf/Protocol, plus ConnLessProtocol, and the client side) with a short responsibility note per class, member names as declared in the headers. Also upgraded to the full AGPL header per your comment on #3790.

Is that the level of detail you had in mind, or do you want it to go deeper (e.g. per-class method walkthroughs)? My instinct is to keep it at "what state lives where and who calls whom" and let the code carry the rest, so the doc doesn't rot.

[pljones]

Whilst keeping this as the "this is the current architecture" guide, I'd like somewhere that directs people on what the "target architecture" is: the clear separation of concerns as raised in #3801. I guess that Issue can have that document as a deliverable.