feat: add unix socket support (#66)

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:

`````markdown
### 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
```
`````

Author: cablehead