Add Changed Block Tracking (CBT) support

[doccaz]

Summary

CBT turned out to need no NFC reverse engineering at all: VirtualMachine.QueryChangedDiskAreas — the actual changed-byte-range query real backup tools use, distinct from VDDK's own VixDiskLib_QueryAllocatedBlocks (a different, disk-internal allocated-block bitmap) — is public VIM API with no VDDK/NFC involvement whatsoever.

Adds thin wrappers:

• enable_change_tracking — sets changeTrackingEnabled via ReconfigVM_Task
• disk_change_id — reads a disk's current changeId off its backing, once CBT is active and a snapshot exists
• query_changed_disk_areas — the actual changed-range query, converted to plain dataclasses

Also documents real-world characteristics found through live testing that aren't obvious from the API docs:

• A wildcard changeId="*" query (the initial-full-backup path) reports only allocated regions on a thin-provisioned disk, not the full sparse virtual capacity.
• A single large contiguous write comes back as one extent regardless of size; only the number of discontiguous changed regions drives how large the extent list gets — the real scaling risk for CBT on a busy disk is fragmented random I/O, not big sequential writes.

Full workflow and lab evidence: docs/cbt.md.

Test plan

• New unit tests (ValueError paths, VIM call argument/result conversion, mocked — no lab needed): pytest tests/unit/test_nfc_auth.py
• New integration test exercising the full cycle: enable CBT, snapshot, write a known sector, snapshot, query, verify the write falls inside the reported extent
• Validated against a live standalone ESXi 8.0.3 host

Note on PR sequencing

Last of 4 focused PRs splitting up the original combined PR #2 (now closed). Zero code overlap with the other three (direct-ESXi fix, GetInfo+DDB_GET, QueryAllocatedBlocks) — pure VIM API, no NFC touched at all. Can merge independently in any order. (The integration test's write step uses the existing NFC write path, so it was validated live by temporarily stacking on the direct-ESXi fix locally, since our test lab is a bare ESXi host that needs that fix to connect at all — the CBT code itself has no such dependency.)

🤖 Generated with Claude Code

[petrutlucian94]

I was a bit surprised when I saw this PR since it's already covered by the public QueryChangedDiskAreas method.

I'm not against having this small wrapper in OpenVixDiskLib though, especially since it was marked as a missing feature / TODO.