The Nim core

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# The Nim core

The program. It owns the state, the screens, the IRC connection and the
message signing; Flutter is a renderer over the widget tree it emits, and
`dart/frq_core` is the FFI binding between them.

```
src/frq_core.nim        the C ABI: every exported symbol, and nothing else
src/frq/ui.nim          the widget tree, in the screens' own tag vocabulary
src/frq/cells.nim       every piece of state the screens read
src/frq/reducer.nim     every event they can send, and what it does
src/frq/model.nim       what a message and a room are
src/frq/rooms.nim       the list, the overview, and the read marker
src/frq/members.nim     who is in a channel, and what the server says of them
src/frq/conn.nim        the socket, the TLS, the line framing
src/frq/ircparse.nim    the IRC wire format
src/frq/handshake.nim   CAP and the SASL exchange inside it
src/frq/atproto.nim     handle → DID → PDS → session
src/frq/crypto.nim      OpenSSL, bound: SHA-256 and Ed25519
src/frq/msgsig.nim      signing a mutation so freeq will accept it
src/frq/emoji.nim       1,884 emoji — generated, see below
src/frq/glyphs.nim      a line split into words and pictures
src/frq/clock.nim       server time in the reader's own zone
src/frq/store.nim       what survives a restart
src/frq/screens/        connect, chats, chat, discover, settings
```

## The ABI

Strings in, strings out, and JSON where the answer is not a single string —
deliberately, against a struct-based ABI: a struct means both sides agreeing
on a memory layout, and a field added later is a version skew that segfaults
instead of failing.

Two rules, both of which cost a segfault to rediscover:

* Every string the core returns is the **caller's** to free, with `frq_free`.
  Nim's allocator is not Dart's.
* `frq_init` exists and does nothing. It used to call `NimMain`, and on Linux
  `--app:lib` already runs the module initialisers from a library constructor
  — so calling it again ran every module's top-level code a second time, which
  for `conn.nim` meant `open()` on channels that were already open.

## Things that will bite

* **Nothing is shared with the socket threads.** There are two — one reader,
  one writer — and they speak only in channels. `recvLine(timeout)` does not
  time out on a TLS socket, so a single thread cannot both wait for the server
  and notice what the client wants to say.
* **`--mm:orc`, not refc.** refc gives each thread its own GC heap, so the
  `Socket` the two threads share is a ref from another heap and dereferencing
  it segfaults.
* **Every intra-package import says `frq/…`.** `import conn` and
  `import frq/conn` name the same file by two paths and Nim compiles it twice,
  giving two sets of globals and two states.
* **`emoji.nim` is generated.** It was `tools/emoji2nim.py` reading the
  ClojureDart catalogue, which is gone; regenerating it now means going back to
  Unicode's `emoji-test.txt` and filtering to what the Twemoji pack draws.
* **The crypto is bindings, not implementations.** `crypto.nim` is OpenSSL
  through EVP. The RFC 8032 vectors in `tests/tcrypto.nim` check that this
  drives it correctly — the seed in the right place, the one-shot signing form,
  the bytes out in the right order — and not that Ed25519 is implemented
  correctly, which is OpenSSL's problem.

```bash
just test nim              # the whole suite
just test nim tircparse    # one file
just build lib             # libfrqcore.so into build/nim
```

## What is not here

`frq.profile` and `frq.replies` were never ported and went with the
ClojureDart rather than moving: a Bluesky profile behind a nick, and asking
freeq what a collapsed msgid was. Neither had a screen in this app to appear
on.

The profile card is state without a screen: `profileViewing` is moved by
nothing and rendered by nothing, and `frq.profile` was never ported.

The emoji picker, the overview strip and the lightbox used to be in that list.
They have screens now — the picker under the message it is for, the overview
as a pane above the compose bar, the lightbox as a panel over the
conversation — so the 🙂 chip, the Overview toggle and clicking a picture all
do what they look like they do.

Untested against a real account: nobody has watched freeq accept a SASL
challenge response or a signature. The shapes are checked and the curve is
OpenSSL's, but the server has not said yes.