feat: auto-assign free port when setPort(0) is called

[Copilot]

setPort() with no arguments (port=0) previously had no effect — the server would not listen on any port. This adds OS-level free-port auto-assignment by binding to port 0, then retrieving the actual assigned port via getsockname().

Semantic changes

• port/https_port default: 0 → -1 — -1 now means "not configured"; 0 means "let OS assign a free port" (consistent with standard socket API convention)
• setPort() ssl_port default: 0 → -1 — prevents setPort(8080) from inadvertently resetting https_port
• start() return value — now returns the actual HTTP port number on success (was always 0), enabling int real_port = server.start()

Usage

server.setHost("127.0.0.1");
server.setPort();               // port=0: OS picks a free port
server.setThreadNum(4);
int real_port = server.start(); // returns the actual assigned port

Affected files

• http/server/HttpServer.h — default port sentinel, setPort() signature, start() return value + doc comment
• http/server/HttpServer.cpp — condition port > 0 → port >= 0; getsockname() call after Listen(0, ...); #include "hsocket.h" added; error logged on getsockname failure
• examples/httpd/httpd.cpp — "no port configured" guard: == 0 → < 0
• docs/cn/HttpServer.md — updated signatures and start() description