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/obf_randchars.go

@@ -0,0 +1,48 @@
+package device
+
+import (
+	"crypto/rand"
+	"strconv"
+	"unicode"
+)
+
+const chars52 = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+
+func newRandCharObf(val string) (obf, error) {
+	length, err := strconv.Atoi(val)
+	if err != nil {
+		return nil, err
+	}
+
+	return &randCharObf{
+		length: length,
+	}, nil
+}
+
+type randCharObf struct {
+	length int
+}
+
+func (o *randCharObf) Obfuscate(dst, src []byte) {
+	rand.Read(dst[:o.length])
+	for i := range dst[:o.length] {
+		dst[i] = chars52[dst[i]%52]
+	}
+}
+
+func (o *randCharObf) Deobfuscate(dst, src []byte) bool {
+	for _, b := range src[:o.length] {
+		if !unicode.IsLetter(rune(b)) {
+			return false
+		}
+	}
+	return true
+}
+
+func (o *randCharObf) ObfuscatedLen(n int) int {
+	return o.length
+}
+
+func (o *randCharObf) DeobfuscatedLen(n int) int {
+	return 0
+}