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.go

@@ -0,0 +1,140 @@
+package device
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+type obfBuilder func(val string) (obf, error)
+
+var obfBuilders = map[string]obfBuilder{
+	"b":  newBytesObf,
+	"t":  newTimestampObf,
+	"r":  newRandObf,
+	"rc": newRandCharObf,
+	"rd": newRandDigitsObf,
+	"d":  newDataObf,
+	"ds": newDataStringObf,
+	"dz": newDataSizeObf,
+}
+
+type obf interface {
+	Obfuscate(dst, src []byte)
+	Deobfuscate(dst, src []byte) bool
+	ObfuscatedLen(srcLen int) int
+	DeobfuscatedLen(srcLen int) int
+}
+
+type obfChain struct {
+	Spec string
+	obfs []obf
+}
+
+func newObfChain(spec string) (*obfChain, error) {
+	var (
+		obfs []obf
+		errs []error
+	)
+
+	remaining := spec[:]
+	for {
+		start := strings.IndexByte(remaining, '<')
+		if start == -1 {
+			break
+		}
+
+		end := strings.IndexByte(remaining[start:], '>')
+		if end == -1 {
+			return nil, errors.New("missing enclosing >")
+		}
+		end += start
+
+		tag := remaining[start+1 : end]
+		parts := strings.Fields(tag)
+		if len(parts) == 0 {
+			errs = append(errs, errors.New("empty tag"))
+			remaining = remaining[end+1:]
+			continue
+		}
+
+		key := parts[0]
+		builder, ok := obfBuilders[key]
+		if !ok {
+			errs = append(errs, fmt.Errorf("unknown tag <%s>", key))
+			remaining = remaining[end+1:]
+			continue
+		}
+
+		val := ""
+		if len(parts) > 1 {
+			val = parts[1]
+		}
+
+		o, err := builder(val)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("failed to build <%s>: %w", key, err))
+			remaining = remaining[end+1:]
+			continue
+		}
+
+		obfs = append(obfs, o)
+		remaining = remaining[end+1:]
+	}
+
+	if len(errs) > 0 {
+		return nil, errors.Join(errs...)
+	}
+
+	return &obfChain{
+		Spec: spec,
+		obfs: obfs,
+	}, nil
+}
+
+func (c *obfChain) Obfuscate(dst, src []byte) {
+	written := 0
+	for _, o := range c.obfs {
+		obfLen := o.ObfuscatedLen(len(src))
+		o.Obfuscate(dst[written:written+obfLen], src)
+		written += obfLen
+	}
+}
+
+func (c *obfChain) Deobfuscate(dst, src []byte) bool {
+	dynamicLen := len(src) - c.ObfuscatedLen(0)
+
+	written, read := 0, 0
+
+	for _, o := range c.obfs {
+		deobfLen := o.DeobfuscatedLen(dynamicLen)
+		obfLen := o.ObfuscatedLen(deobfLen)
+
+		if !o.Deobfuscate(dst[written:written+deobfLen], src[read:read+obfLen]) {
+			return false
+		}
+
+		written += deobfLen
+		read += obfLen
+	}
+
+	return true
+}
+
+func (c *obfChain) ObfuscatedLen(n int) int {
+	total := 0
+	for _, o := range c.obfs {
+		total += o.ObfuscatedLen(n)
+	}
+	return total
+}
+
+func (c *obfChain) DeobfuscatedLen(n int) int {
+	dynamicLen := n - c.ObfuscatedLen(0)
+
+	total := 0
+	for _, o := range c.obfs {
+		total += o.DeobfuscatedLen(dynamicLen)
+	}
+	return total
+}