fix(internet): MH mobility options that do not round-trip after a one-octet change #2684

Found while reviewing #2683 and confirmed by a Sonnet cross-review. Unchanged by that PR. Each case below is a one-octet change to an option inside the BRR in options-internet.pcap (offsets are within the option), and each breaks the owner's rule that a parse→rebuild round trip matches byte for byte:

• Length 0 rewritten: MN-ID (8) with Option Length 0 rebuilds 0x11. LMA-Controlled MAG Parameters (62) and Access Network Identifier (52) with a sub-option length of 0 rebuild 0x06 and 0x0d.
• Parse accepts, rebuild refuses: a prefix length out of range parses, then raises invalid prefix length on rebuild. Affected: IPv4 Address Acknowledgement (30), IPv4 Home Address Request (36), IPv4 Home Address Reply (37) and Delegated MNP (55).
• Service Selection (20): non-UTF-8 text grows the message from 16 to 24 octets on rebuild.
• Quality of Service (58): a bit set at offset 3 is dropped.
• CGA Parameters (12): changing offset 28 rebuilds 56 octets as 168, and changing offset 34 rebuilds them as 48.
• Option type changed: option 13 retyped as 12 rebuilds 16 octets as 40. Option 42 retyped as 0 rebuilds a different octet.

Repro scripts are in the review's scratch tree, /tmp/2669r/scripts/c6_side.py. Each reader should keep the octets it was given (or refuse, where the RFC says to discard), so the rebuild writes them back. Fixes per #2669's pattern and the short-read convention.