lx: fix transport padding buffer overrun + harden AWG config guards

Transport padding (s4) crashed the whole process with
"index out of range" in RoutineSequentialSender on the first data
packet: InputPacket/InputPackets sized elem.buffer without headroom
for the in-buffer right-shift that prepends the random prefix.

- send.go: reserve paddings.transport in both injection-path
  allocLength computations; replace the manual backward byte loop
  with an overlap-safe copy; defensively grow the buffer (pool-backed)
  if it still lacks headroom, dropping packets that cannot fit a
  single WG message instead of overrunning.
- receive.go: drop the rxBytes/timers block duplicated by the AWG
  re-graft (rx accounting was doubled, keepKeyFreshReceiving fired
  twice per batch).
- send.go: swap jmin/jmax when configured inverted (UAPI validates
  the fields only individually; a swapped pair panicked rand.Int
  with a non-positive bound on the first handshake).
- obf*.go: bound obfuscator length args to [0, MaxMessageSize]
  (negative panicked slice bounds, huge ones OOMed the handshake).
- magic-header.go: widen to int64 before end-start+1 so a full-range
  header cannot wrap to a zero rand.Int bound.

Tests: transport_padding_test.go reproduces the on-device crash
byte-for-byte (red on the previous commit, green now) across both
injection paths and the tun path; obf_guards_test.go pins the
config-value guards.

device/send.go

@@ -215,6 +215,11 @@ func (peer *Peer) SendHandshakeInitiation(isRetry bool) error {
 	jc := peer.device.junk.count
 	jmin := peer.device.junk.min
 	jmax := peer.device.junk.max
+	if jmax < jmin {
+		// UAPI validates jmin/jmax only individually; a swapped pair
+		// would panic rand.Int below with a non-positive bound.
+		jmin, jmax = jmax, jmin
+	}
 
 	for i := 0; i < jc; i++ {
 		nBig, _ := rand.Int(rand.Reader, big.NewInt(int64(jmax-jmin+1)))
@@ -518,7 +523,9 @@ func (device *Device) InputPacket(destination []byte, packetSlices [][]byte) {
 	for _, packetSlice := range packetSlices {
 		totalLength += len(packetSlice)
 	}
-	allocLength := MessageEncapsulatingTransportSize + MessageTransportHeaderSize + totalLength + PaddingMultiple + chacha20poly1305.Overhead
+	// paddings.transport (AWG s4) is prepended in-buffer by
+	// RoutineSequentialSender; reserve headroom for the shift.
+	allocLength := MessageEncapsulatingTransportSize + MessageTransportHeaderSize + totalLength + PaddingMultiple + chacha20poly1305.Overhead + device.paddings.transport
 	if allocLength > MaxMessageSize {
 		return
 	}
@@ -564,7 +571,9 @@ func (device *Device) InputPackets(packets []*InputPacketRef) []*InputPacketRef
 		for _, packetSlice := range packetRef.PacketSlices {
 			totalLength += len(packetSlice)
 		}
-		allocLength := MessageEncapsulatingTransportSize + MessageTransportHeaderSize + totalLength + PaddingMultiple + chacha20poly1305.Overhead
+		// paddings.transport (AWG s4) is prepended in-buffer by
+		// RoutineSequentialSender; reserve headroom for the shift.
+		allocLength := MessageEncapsulatingTransportSize + MessageTransportHeaderSize + totalLength + PaddingMultiple + chacha20poly1305.Overhead + device.paddings.transport
 		if allocLength > MaxMessageSize {
 			continue
 		}