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/magic-header.go

@@ -57,7 +57,9 @@ func (h *magicHeader) Validate(val uint32) bool {
 }
 
 func (h *magicHeader) Generate() uint32 {
-	high := int64(h.end - h.start + 1)
+	// Widen before arithmetic: end-start+1 in uint32 wraps to 0 for the
+	// full 0..2^32-1 range, which would panic rand.Int (bound <= 0).
+	high := int64(h.end) - int64(h.start) + 1
 	r, _ := rand.Int(rand.Reader, big.NewInt(high))
 	return h.start + uint32(r.Int64())
 }