lx: re-graft AmneziaWG 2.0 obfuscation onto sagernet/wireguard-go v0.0.3

Migrate the AmneziaWG graft from the old sagernet base (506b763) onto v0.0.3,
the base sing-box 1.14 uses. Strategy: start from v0.0.3, copy the net-new
obfuscation files verbatim from the proven graft (27290b6), take the 6 modified
device/ files wholesale from the graft, keep all tun/* and conn/* from v0.0.3.

Net-new (copied from 27290b6): device/magic-header.go + device/obf*.go (9).
Modified (from 27290b6): device/{send,receive,uapi,device,noise-protocol,cookie}.go.
  - receive.go: reverted two Go-1.22 'range int' loops to the v0.0.3 form (go 1.20).
  - cookie.go: keeps the 4-arg CreateReply(msgType) the grafted send.go calls.
  - noise-protocol.go: MessageEncapsulatingTransportSize=0 (AWG composes without
    the sagernet Bind.Send headroom prepend).

§010 android UDP_GRO guard (fb8d8d8) intentionally DROPPED: v0.0.3 fixes the
receive split-brain at source — bind_std.go now gates all 5 RX/offload paths on
linux||android, and gso_linux.go splits coalesced packets on android. Re-inserting
the guard would disable a now-working android RX-offload path. Requires on-device
re-verification before release.

go.mod: go 1.20, golang.org/x/sys v0.21.0 (v0.0.3 baseline). No amnezia-vpn import
paths remain. Builds clean for linux/android/windows/darwin (library packages).

device/magic-header.go

@@ -0,0 +1,63 @@
+package device
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"math/big"
+	"strconv"
+	"strings"
+)
+
+type magicHeader struct {
+	start uint32
+	end   uint32
+}
+
+func newMagicHeader(spec string) (*magicHeader, error) {
+	parts := strings.Split(spec, "-")
+	if len(parts) < 1 || len(parts) > 2 {
+		return nil, errors.New("bad format")
+	}
+
+	start, err := strconv.ParseUint(parts[0], 10, 32)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse %s: %w", parts[0], err)
+	}
+
+	var end uint64
+	if len(parts) > 1 {
+		end, err = strconv.ParseUint(parts[1], 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse %s: %w", parts[1], err)
+		}
+	} else {
+		end = start
+	}
+
+	if end < start {
+		return nil, errors.New("wrong range specified")
+	}
+
+	return &magicHeader{
+		start: uint32(start),
+		end:   uint32(end),
+	}, nil
+}
+
+func (h *magicHeader) GenSpec() string {
+	if h.start == h.end {
+		return fmt.Sprintf("%d", h.start)
+	}
+	return fmt.Sprintf("%d-%d", h.start, h.end)
+}
+
+func (h *magicHeader) Validate(val uint32) bool {
+	return h.start <= val && val <= h.end
+}
+
+func (h *magicHeader) Generate() uint32 {
+	high := int64(h.end - h.start + 1)
+	r, _ := rand.Int(rand.Reader, big.NewInt(high))
+	return h.start + uint32(r.Int64())
+}