[JarryShaw]

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• MPLOption.seed in the HOPOPT and IPv6-Opts schemas said int, but for
  seed type 0 post_process stores the IPv6 source address. Annotate it
  'int | IPv6Address', and the __init__ stub's seed
'Optional[int | IPv6Address]', matching the data models.
• Add a regression test pinning both annotations and the runtime types: an
  IPv6 packet whose HOPOPT or IPv6-Opts MPL option has S=0 yields an
  IPv6Address seed, and S=1 to 3 yield an int.

Closes #1840

[JarryShaw]

Verdict: GOOD TO GO at 860aeeabe. A Sonnet cross-review (author: Opus) confirmed:

• The runtime behaviour, by parsing: S=0 gives IPv6Address('2001:db8::1') and S=1–3 give int, in both HOPOPT and IPv6-Opts.
• The annotation tests fail on main and pass here. The runtime test passes on both, because it characterises behaviour that was already correct.
• mypy output on the schema and protocol files is byte-identical to main.
• Nothing in pcapkit evaluates schema annotations at runtime, and the same files already carry string unions.

Out of scope, pre-existing: a standalone HOPOPT(bytes) with S=0 gives seed is NO_VALUE. Sibling fields leave the sentinel out of their annotations too, and the consumer at hopopt.py:1137 already handles it.