Refactor UFFD

[ValentaTomas]

This should be only a refactor now—I tried to minimize all functional changes.

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Note

Refactors UFFD into a dedicated userfaultfd type with fd helpers, replaces old mapping with a tested memory mapping, and updates integration/tests accordingly.

• Userfaultfd refactor:
  • Introduce userfaultfd.Userfaultfd with Serve/Close, encapsulating polling, pagefault handling, and missing-page copy logic.
  • Add userfaultfd/fd.go wrapping UFFD syscalls/constants; remove old constants.go and syscalls.go.
  • Move eagain logging to userfaultfd and streamline read/poll loop.
• Memory mapping module:
  • Replace mapping package with memory module: Region, Mapping, GetOffset, and AddressNotFoundError.
  • Add comprehensive tests in memory/mapping_test.go.
• Orchestrator integration:
  • Update uffd.Uffd to parse region mappings, construct memory.Mapping, create userfaultfd.Userfaultfd, and call Serve.
  • Remove legacy Serve implementation and mapping types; rename buffers (regionMappings*).
• Tests:
  • Migrate tests to userfaultfd package; update cross-process helpers to new fd API and mapping; adjust helpers and missing/missing_write tests.

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Should be addressed now.