fix(mh): Message ID option always writes and reads a zero fraction

[JarryShaw]

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Closes #1093

_make_opt_mesg_id now writes min(round(frac * 2**32), 0xFFFFFFFF). MesgIDOption.post_process now adds fraction / 2**32 seconds. timestamp is still a UTC datetime, and it now has sub-second precision. The unused math import is dropped from the schema module.

Probe at 1_700_000_000.5, before → after: fraction 2147483648000000 → 2147483648, bytes 0a08e8fe6f8000000000 → 0a08e8fe6f8080000000, read 22:13:20 → 22:13:20.500000.

Tests (new tests/protocols/internet/test_mh_mesg_id_fraction_unit.py): fractions 0, 0.5, 0.999999, an exact second, the clamp and the read scale.

• With the fix: 4 passed, 3 subtests passed. Fix reverted: 4 failed, 2 passed (fraction 0 and the exact second pass either way).
• test_mh_unit.py 52 passed, test_option_roundtrip_unit.py 6 passed, tests/project 379 passed, 1 skipped.

[JarryShaw]

Cross-review verdict on 1d1348342: GOOD TO GO (ran on Sonnet; author Opus)

• Encoding: the fraction is written as round(frac*2**32), clamped to 32 bits, and read back as fraction/2**32 seconds. That matches the 32.32 NTP format in RFC 4285 §5, which mh.py cites.
• Probe: 1_700_000_000.5 now packs …80000000 and reads back as .500000.
• Round-trip: 20,000 random microsecond values all round-trip on this PR, and every one mismatches on main. Epoch 0 and pre-1970 values are also correct.
• Tests: with the source reverted, 4 tests fail. test_mh_unit (52), test_option_roundtrip_unit and test_option_coverage_runtime pass.
• Public field: timestamp is still a UTC datetime. Only its sub-second part changes.

Nit, pre-existing: the MesgIDOption.seconds comment says the epoch is 1970, but NTP counts from 1900. The code is right.

[github-actions]

Coverage: 84.75% (unit tier, Python 3.14, 1d1348342, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	4	20	4	93.55%
pcapkit/const	18757	1048	2342	855	90.11%
pcapkit/corekit	1874	160	578	35	89.85%
pcapkit/dumpkit	136	25	40	6	76.70%
pcapkit/foundation	2422	312	842	82	84.19%
pcapkit/interface	112	7	40	5	92.11%
pcapkit/protocols	15652	1196	3942	366	89.91%
pcapkit/toolkit	487	109	144	9	74.64%
pcapkit/utilities	429	4	122	4	98.55%
pcapkit/vendor	4409	2359	1006	158	42.84%

Per-file detail: the coverage-html artifact of this run.