release-0-20: Parse & encode the IPv6 fragment header in network byte order

[JulianSchmid]

Cherry pick of #149 to 0.20 release branch

The 13-bit fragment offset and the "more fragments" (M) flag were encoded/decoded with the wrong bit positions: the offset's low bits were packed into byte 3 bits 4..0 and M was read from byte 3 bit 7. Per RFC 8200 §4.5, bytes 2..3 form a big-endian 16-bit word where the offset is the top 13 bits (word >> 3) and M is the least significant bit.

The encode and decode bugs were symmetric, so to_bytes() round-trips parsed back cleanly and every existing test passed — but the bytes on the wire did not match the RFC or any other stack. A hand-crafted byte-level test (network_byte_order) is added to break that symmetry and lock the layout.

The bit layout is now defined in a single place (Ipv6FragmentHeaderSlice); read/read_limited and is_fragmenting_payload delegate to it so the three copies can no longer drift apart.

Note: this changes the serialized bytes for any header with a non-zero offset or the M flag set, so it is a wire-format-breaking fix.

(cherry picked from commit 6889de9)

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