feat(usockets): optimize high concurrency support up to 10,000+ connections

- Increase uSockets listen() backlog from 512 to 16384 in bsd.c
- Add high concurrency test suite (5000-10000 concurrent connections)
- Add HIGH_CONCURRENCY.md documentation with optimization guide
- Add usockets/build.sh script for rebuilding uSockets library
- Update README with latest benchmark results

Performance (macOS M3 Max):
- 5,000 concurrent: 81,524 RPS, 100% success
- 10,000 concurrent: 24,198 RPS, 100% success

System requirements:
- kern.ipc.somaxconn >= 8192
- ulimit -n 65535

usockets/usockets.v

@@ -105,6 +105,8 @@ pub fn (ctx SocketContext) on_writable(h voidptr) { C.us_socket_context_on_writa
 pub fn (ctx SocketContext) on_timeout(h voidptr) { C.us_socket_context_on_timeout(0, ctx, h) }
 pub fn (ctx SocketContext) on_end(h voidptr) { C.us_socket_context_on_end(0, ctx, h) }
 pub fn (ctx SocketContext) listen(port int) ListenSocket {
+	// 参数: ssl=0, context, host=nil, port, options=LIBUS_LISTEN_DEFAULT(0), socket_ext_size=0
+	// 注意: backlog 是在 uSockets 库内部硬编码的，不通过此函数传递
 	return C.us_socket_context_listen(0, ctx, unsafe { nil }, port, 0, 0)
 }