Refactor dirty page tracking

[ValentaTomas]

Note

Refactors dirty-page tracking to a new block Tracker, updates UFFD to unregister and return tracked pages via Disable(ctx), and wires changes through sandbox, build diffs, and tests.

• Memory/UFFD:
  • Replace internal missing/dirty tracking with block.Tracker; add Disable(ctx) (*block.Tracker, error) and remove direct Dirty() from MemoryBackend.
  • uffd.New drops block-size param; Uffd.Disable now unregisters mappings (adds UFFDIO_UNREGISTER) and returns tracker; internal dirty(ctx) helper added.
  • Enforce page-size consistency in userfaultfd.NewUserfaultfdFromFd; limit in-flight requests to 4096.
• Block:
  • Add Tracker with Add/Has/Reset/Clone/Offsets/BitSet/BlockSize; Slicer now requires BlockSize().
  • Remove old tracked slice device; new bitsetOffsets and iter-based offsets.
• Sandbox Integration:
  • Use memory.Disable(ctx) to get dirty pages; pass dirty.BitSet() to memory diff; update serveMemory to new uffd.New signature.
• Build/Diff:
  • Add BlockSize() to Diff implementations (NoDiff, localDiff, StorageDiff).
• Tests/Utils:
  • Add tracker_test.go; revise cross-process test to stream Tracker.Offsets(); add utils.TransformTo iterator helper; test MemorySlicer now implements BlockSize().

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[ValentaTomas]

I'll do some last end-to-end testing and then we should be able to merge.