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/obf_datasize.go

@@ -1,9 +1,7 @@
 package device
 
-import "strconv"
-
 func newDataSizeObf(val string) (obf, error) {
-	length, err := strconv.Atoi(val)
+	length, err := parseObfLen(val)
 	if err != nil {
 		return nil, err
 	}