fix(ipv6-ext): IPv6_Ext rebuild drops the header body, and an overrun writes Hdr Ext Len 0xff #1230

Describe the bug
IPv6_Ext._make_data (ipv6_ext.py:617-645) passes only cls._make_payload(data), the upper layer, so the opaque body after the first two octets is never emitted. The length octet comes from data.length, which is then wrong. With next=None (overrun) data.length // 8 - 1 is -1 and is written as ff.

To Reproduce
IPv6_Ext(io.BytesIO(b), len(b), extension=True, alias=140), then IPv6_Ext.from_data(p.info).data.hex(): 3b00010203040506 -> 3b00; 3b01 + 14 octets -> 3b01; a bare 3b00 (needs 8) -> 00ff. Through IPv6 with next=140 the 8-octet header rebuilds as 2.

Expected behavior
The body is recorded and re-emitted so the rebuild is byte-exact; an overrun is not written as -1.

Found by the internet-v6 round-trip audit (#1202); reproduced on main at 11ae68ebf (repro check D3).

Found by the internet-v6 round-trip audit (#1202). The repro was re-checked by an independent verifier, and I re-ran it on main at 11ae68ebf.