feat: add unix socket support

[cablehead]

This PR also removes magic terminology since we no longer refer to the endpoint as a magic endpoint.

Unit tests have been added. For a manual smoke test:

### 1. Stand up a dummy Unix-socket “echo” server

This will listen on `/tmp/backend.sock` and echo back whatever you send it:

```bash
socat UNIX-LISTEN:/tmp/backend.sock,reuseaddr,fork EXEC:'/bin/cat'
```

Leave that running in Terminal A.

---

### 2. Run your new `listen-unix` side

Point it at the socket you just created:

```bash
dumbpipe listen-unix --socket-path /tmp/backend.sock
```

You’ll see something like:

```
Forwarding incoming requests to '/tmp/backend.sock'.
To connect, use e.g.:
dumbpipe connect-unix --socket-path /tmp/client.sock <NODE_TICKET>
```

Copy the `<NODE_TICKET>`.

---

### 3. Run your new `connect-unix` side

In Terminal C, do:

```bash
dumbpipe connect-unix --socket-path /tmp/client.sock <NODE_TICKET>
```

Now you’ve got a local socket at `/tmp/client.sock` that—behind the scenes—is piping straight through to your echo server.

---

### 4. Verify it works

In Terminal D, send a test string through the client socket and watch it come back:

```bash
echo "hello world" | socat - UNIX-CONNECT:/tmp/client.sock
# you should see: hello world
```

[Arqu]

clippy got a bump late last night :/

[rklaehn]

Thanks. Looks good on first glance.

[rklaehn]

I think we should remove magic terminology from this, since we no longer refer to the endpoint as a magic endpoint.

[cablehead]

I made a pass through, and removed references to magic terminology

1/ hope matches matches what you were thinking?
2/ hopefully it's not overloading this patch?

[cablehead]

I have been thinking magicpipe would be a better name than dumbpipe for this utility though. It really feels like magic!

[rklaehn]

Very nice. Many of the code changes are just to make clippy happy and generic DRY. The code itself is nicely isolated. I don't see why we wouldn't merge this.

One thing: it would be nice to have instructions how to test this locally. I just want to give it a very basic manual smoke test.

[cablehead]

clippy got a bump late last night :/

I can't for the life of me work out how CI is using clippy, I can't see these locally :) It might just take a few pushes to catch them all 🙏

[cablehead]

One thing: it would be nice to have instructions how to test this locally. I just want to give it a very basic manual smoke test.

I've updated the README to document the new commands.

This is a quick screencast I posted to the discord running a smoke test:

dumbpipe-unix.mp4

[cablehead]

Hoping that last push will address the flakiness 🤞

[cablehead]

Looks like a flake in connect_listen_ctrlc_connect? (unrelated to this PR)

[Arqu]

Yup seems to have flaked. @rklaehn Can I ask you to do a one over on this.

[cablehead]

Anything I can do to help this one get over the line?

[rklaehn]

Sorry about dropping the ball on this a bit. I was traveling a lot.

The code seems to be pretty well separated, doesn't add any additional deps, so works for me.

Not so sure about the zellij example as an example, maybe we can come up with something more lightweight. But that can happen in a subsequent PR.

[cablehead]

Thanks for the merge!

Not so sure about the zellij example as an example, maybe we can come up with something more lightweight. But that can happen in a subsequent PR.

That makes sense. If you think of a lighter example, tag me in an issue and I can PR. I'll try and think of a lighter one too.