fix(ipv6): from_data drops the extension-header chain from the rebuild #1177

IPv6.from_data(ipv6.info) drops the extension-header chain. Found during the #1176 cross-review. It reproduces on origin/main and is not caused by #1176.

Repro (at 222dd72a2): an IPv6 header with next header 43 and an 8-octet Routing header is 48 octets on the wire. The rebuild is 40 octets, with the Routing header gone. With a 12-octet extension it is 52 octets on the wire and 44 rebuilt: the 8-octet header is lost and only the trailing Raw survives.

Scope: of the 183 non-exact frames in the capture from_data sweep, up to 134 have an extension header in the chain:

134 is an upper bound, because no frame was isolated to a single cause. The IPv6_Frag frames are probably a separate misparse. The other 49 are IPv6>HIP frames from options-internet.pcap, which this does not explain.

Expected: a rebuild of a parsed IPv6 packet keeps its extension headers, byte for byte.