Refactor dirty page tracking

[ValentaTomas]

Note

Replaces dirty page tracking with a thread-safe Tracker and updates UFFD, sandbox, and build components to use it, adding unregister support and minor API changes.

• Block / Tracking:
  • Introduce block.Tracker (thread-safe bitset) with Has, Add, Reset, Offsets, BitSet, Clone; unit tests added.
  • Extend Slicer with BlockSize().
• UFFD:
  • Change constructor to uffd.New(memfile, socketPath); remove TrackedSliceDevice usage.
  • MemoryBackend API: Disable(ctx) now returns *block.Tracker.
  • Implement unregister via UFFDIO_UNREGISTER; Uffd.Disable unregisters then returns Tracker from Dirty().
  • Track faults via block.Tracker with bounded concurrency; expose offsets iterator in cross-process tests.
  • Buffer Ready() channel.
• Sandbox Integration:
  • On pause, call s.memory.Disable(ctx) and pass dirty.BitSet() to MemoryDiffCreator.
  • serveMemory updated for new UFFD constructor.
• Build / Diff:
  • Add BlockSize() to NoDiff, localDiff, and StorageDiff.
• Utils:
  • Add utils.TransformTo iterator helper.

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[dobrac]

lgtm in general, just few nits

[ValentaTomas]

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