Extract low level UFFD handler to a separate package

[ValentaTomas]

• Extract low level UFFD handler to a separate package
• Remove unused struct
• Improve uffd tests (ensure they are not flaky)
  • Add the 2M hugpages tests

---

Note

Extracts low-level UFFD into a new userfaultfd package, replaces mapping with memory map API, wires into orchestrator, and adds robust 4K/2MB tests; CI enables UFFD and hugepages.

• Orchestrator UFFD refactor:
  • Move low-level handling to new userfaultfd package (Serve, configureApi, Register, copy, Close).
  • Replace mapping with memory package (MemfileMap, MemoryMap.GetOffset); update Firecracker mapping and JSON parsing.
  • Update uffd integration to use userfaultfd.NewUserfaultfdFromFd(...).Serve(...); remove TrackAndReturnNil.
  • Simplify UFFD constants/APIs (remove write-protect helpers; adjust pagefault address/dst handling).
• Tests:
  • Add userfaultfd tests covering 4K and 2MB hugepages; ensure non-flaky behavior.
  • Introduce testutils helpers (NewPageMmap, ContiguousMap, random data, content slicer) and utils.MapKeys.
• CI:
  • Workflow enables unprivileged userfaultfd and mounts hugepages before tests.
• Misc:
  • Minor signature tweak in MaskTemplate.Close.

^{Written by Cursor Bugbot for commit dcea5f1. This will update automatically on new commits. Configure here.}

[ValentaTomas]

I'll improve the tests a little, but the rest should be ready.

[ValentaTomas]

Included in #1385, very hard to merge separately.