Incoming: _parse_sms_concat parses UDH for IEI 0x00 (8-bit ref) and
IEI 0x08 (16-bit ref). PoRSubmitHandler accumulates segments, sorts by
num, and reassembles complete payload when all parts arrive.
Outgoing: MAX_ENVELOPE_SEGMENTS=5 limits the secured packet to 5
ENVELOPE chunks (650B max at 130B/chunk).
10 new tests for concat parsing and reassembly.