fix(schema): Schema.pack drops the end-of-options padding length, so TCP and IPv4 rebuild shorter than their header says #1289

Describe the bug
Nothing on the pack path sets __option_padding__, so PaddingField(length=lambda pkt: pkt.get("__option_padding__", 0)) resolves to 0 (schema.py:822-824 packs bytes(field.length); fields at schema/transport/tcp.py:1148, schema/internet/ipv4.py:716, and the pcapng option padding in schema/misc/pcapng.py, e.g. :917). Only the length is covered here; the padding octet values are #1223, and the unpack-side padding handling in #1229 is a different path.

To Reproduce
Rebuild with TCP.from_schema(tcp.schema.to_dict()):

• tcp.pcap frame 1: 44 -> 43 octets (...04020000 -> ...040200), Data Offset still 11.
• many_interfaces.pcapng frames 24 and 52: the same 44 -> 43.
• IPv4 460000180000000040060000c0a80001c0a8000201000000 (NOP, EOOL, 2 pad): 24 -> 22 octets, IHL still 6.

Repro: /tmp/rtissues/corekit-repro.py (check D3) on origin/main 11ae68e.

Expected behavior
The padding length is carried through to_dict()/from_dict() so the rebuilt header matches its own length field.

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