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 🤞