Add Spinner, a Minecraft hostname-routing reverse proxy

Routes Java Edition connections by the hostname in the handshake, with
wildcard domains, backend failover, PROXY protocol v2, cached server-list
status from health probes, hot-reloaded config, and an admin page and
metrics endpoint. Includes CI, tagged releases and a container image.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/proxy/proxyproto.go

@@ -0,0 +1,31 @@
+package proxy
+
+import (
+	"encoding/binary"
+	"net"
+)
+
+var v2sig = []byte("\r\n\r\n\x00\r\nQUIT\n")
+
+// proxyV2Header builds a PROXY protocol v2 header carrying the client's real
+// address so the backend can see it. Falls back to a LOCAL header if the
+// addresses aren't TCP.
+func proxyV2Header(src, dst net.Addr) []byte {
+	s, sok := src.(*net.TCPAddr)
+	d, dok := dst.(*net.TCPAddr)
+	h := append([]byte{}, v2sig...)
+	if !sok || !dok {
+		return append(h, 0x20, 0x00, 0, 0)
+	}
+	if s4, d4 := s.IP.To4(), d.IP.To4(); s4 != nil && d4 != nil {
+		h = append(h, 0x21, 0x11, 0, 12)
+		h = append(h, s4...)
+		h = append(h, d4...)
+	} else {
+		h = append(h, 0x21, 0x21, 0, 36)
+		h = append(h, s.IP.To16()...)
+		h = append(h, d.IP.To16()...)
+	}
+	h = binary.BigEndian.AppendUint16(h, uint16(s.Port))
+	return binary.BigEndian.AppendUint16(h, uint16(d.Port))
+}