fix(ipv4): TS rebuild floors a float and loses 1 ms on ~1% of timestamps #1391

Describe the bug
_make_opt_ts converts a timedelta to milliseconds with math.floor(ts.total_seconds() * 1000) (pcapkit/protocols/internet/ipv4.py:1603, :1612, :1635, :1652). The float product is inexact (1.001 × 1000 = 1000.9999999999999), so the floor loses 1 ms. The reader (schema/internet/ipv4.py:395) is exact, so parse → rebuild is not byte-exact.

To Reproduce

• The from_data round trip of the TS option 44080900 000003e9 (flag 0, one stamp of 1001 ms) rebuilds as 44080900 000003e8.
• 1001 ms is the smallest failing value. 23,602 of the first 3,000,000 values fail.
• Flag 1 and make(timestamp=[timedelta(milliseconds=1001)]) fail the same way.
• No other total_seconds() site in pcapkit/ has the problem.

Expected behavior
Every 31-bit stamp rebuilds unchanged. Integer arithmetic, td // timedelta(milliseconds=1), gives 1001.

Reproduced on main at 66950b1f2; found by the #1390 cross-review. Blocked until #1390 merges, because both edit _make_opt_ts.