feat: report the port a server is listening on

[sash-a]

Problem

TcpTransport takes a port and tells nobody what it did with it — the field is private on a frozen pyclass, and _Server never exposes its transport. So TcpTransport(port=0) bound an ephemeral port that nothing could discover.

Change

server = Server(example, batch_size=32, transport=TcpTransport(port=0))
server.start()
print(server.port)  # 42201 — publish this to your clients

Server.port raises RuntimeError before start() and without a transport. Transport::port on the Rust trait defaults to None for transports that don't listen on a TCP port.

The one non-obvious part: the listener now binds in start() instead of in the accept task it spawns. The bound address doesn't exist until something calls local_addr, and nothing could from inside a task that had already been handed to the runtime. Moving the bind is what makes the port knowable at all.

One behaviour change falls out of that: a failed bind (port in use) now raises from start(). It previously only reached stderr, with start() returning successfully on a server that was never listening.