Refactor dirty page tracking

[ValentaTomas]

Note

Replace dirty-page tracking with a thread-safe bitset Tracker, add UFFD unregister support, and make memory backend Disable/Dirty context-aware across sandbox and UFFD.

• Block tracking:
  • Introduce block.Tracker (thread-safe bitset) with Add, Has, Reset, BitSet, Clone, BlockSize; add comprehensive tests.
• UFFD integration:
  • Replace sync.Map/TrackedSliceDevice with Tracker for missingRequests; use Add to de-dup faults.
  • Add unregister support via UFFDIO_UNREGISTER and unregister(); expose Userfaultfd.Unregister() and wire through Uffd.Disable(ctx).
  • Update constructors to pass page size; adjust message handling and tests to read offsets from Tracker.
• API changes:
  • Update MemoryBackend to Disable(ctx) and Dirty(ctx) (*block.Tracker, error); migrate NoopMemory and Uffd accordingly.
• Sandbox:
  • Use new memory.Disable(ctx) and memory.Dirty(ctx); pass dirty.BitSet() to memory diff creation.

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[ValentaTomas]

I want to do a few more end to end tests, but it should be ready to merge right after https://github.com/e2b-dev/infra/tree/uffd-refactor