fix(ipv4): SEC rebuild rewrites a non-conforming Field Termination Indicator #1393

Describe the bug
_make_opt_sec sets each Field Termination Indicator from the octet's position (pcapkit/protocols/internet/ipv4.py:1514-1522). _read_opt_sec only warns about a wrong bit (:800) and does not keep the raw octets. So a SEC option carrying a non-conforming termination bit is normalised on rebuild instead of round-tripping. #1321 fixed the inverted convention but not this.

To Reproduce
from_data round trip, option bytes in → out:

• 8204ab91 → 8204ab90
• 82041181 → 82041180
• 8205118001 → 8205118100

Expected behavior
Parse → rebuild is byte-exact, per the standing round-trip rule. The warning can stay, but the wire bits should survive.

Reproduced on main at 66950b1f2; found by the #1390 cross-review. Blocked until #1390 merges, because both edit protocols/internet/ipv4.py.