config: ServerConfig.Tcp defaults to new(), so a server that never asks for TCP still binds 8080

[MDA2AV]

ServerConfig has four nullable option groups, and only one of them arrives already switched on:

public IncrementalOptions? Incremental { get; init; }        // null
public TcpOptions?  Tcp  { get; init; } = new();             // TcpOptions.Port = 8080
public UdpOptions?  Udp  { get; init; } = new();             // UdpOptions.Ports is empty
public QuicOptions? Quic { get; init; }                      // null

Quic and Incremental default to null. Udp does default to new(), but UdpOptions.Ports is empty by default and its own doc says so - "Empty (default) means no UDP sockets are opened" - so the instance is inert and exists only to carry tunables.

TcpOptions is the exception. Its Port defaults to 8080, so the default instance is not inert, it is a live listener. That makes "no TCP" an opt-out rather than an absence, and it is the only transport where forgetting a line binds a port.

Because the default is a property initializer it runs before any object initializer, so a config built as new ServerConfig { ... } keeps the live listener for every property it does not explicitly set. A QUIC-only server has to write Tcp = null and mean it. ServerConfig's own summary already documents the workaround, which is a fair sign the shape is inverted:

Defaults to an enabled listener on TcpOptions.Port; set it to null to open no TCP listener at all, which is what a QUIC-only server wants - otherwise the reactor binds 8080 regardless and inbound connections reach a Reactor.TcpHandle that was never set.

Not hypothetical: the GenHTTP ioxide engine hit exactly this. It builds a ServerConfig and then adds listeners conditionally from the endpoint bindings (WithTcp only when some endpoint asked for HTTP/1 or HTTP/2, WithQuic only when one asked for HTTP/3). An HTTP/3-only server therefore never overwrites Tcp, inherited the default, and bound 8080 for a protocol it does not serve. It needed a commit of its own to start from no listeners, and carries a permanent Tcp = null with a comment explaining why. Worth noting the failure there is worse than the doc warns: that engine sets TcpHandle unconditionally, so a connection to the stray 8080 does not find an unset handler, it reaches a live one that looks the listener port up in a per-port endpoint table and throws KeyNotFoundException.

Two ways out, depending on what the default is meant to buy:

1. Default Tcp to null, so config starts from no listeners and a listener is something you add - consistent with Quic.
2. Keep Tcp = new() but make TcpOptions inert by default the way UdpOptions already is: a ports collection that is empty unless filled. Same end state, and it makes the two listener-owning transports the same shape.

The cost of (1) is the zero-config case: new ServerConfig() no longer gives a working server on 8080, and every consumer relying on the implicit default needs Tcp = new(). That is the actual tradeoff to decide, and it is why this is filed rather than just changed.