Enable dirty-page tracking, take chained diff snapshots of a Firecracker microVM, merge the layers with snapshot-editor, and restore the merged result

Documented steps

1. Enable dirty-page tracking pre-boot: PUT /machine-config {"vcpu_count": 2, "mem_size_mib": 1024, "smt": false, "track_dirty_pages": true} — turns on KVM's dirty-page log so diff snapshots contain exactly the pages written since boot/last snapshot (at some CPU cost).
2. Pause: PATCH /vm {"state": "Paused"}.
3. Create the first diff snapshot: PUT /snapshot/create {"snapshot_type": "Diff", "snapshot_path": "./snapshot_file", "mem_file_path": "./mem_file"}. On success the dirty-page bitmap is reset.
4. Resume (PATCH /vm {"state": "Resumed"}), let the VM run, then repeat pause + Diff /snapshot/create for each additional layer (each contains only pages dirtied since the previous snapshot).
5. Merge layers onto the base IN CREATION ORDER: `snapshot-editor edit-memory rebase --memory-path path/to/base --diff-path path/to/layer` (the deprecated rebase-snap tool also works). After each rebase the base is a resumable memory file as of that layer's time.
6. For the final restore, use the microVM STATE file created in the same /snapshot/create call as the last-merged memory layer — never a state file from a different call.
7. Load in a fresh process: PUT /snapshot/load {"snapshot_path": "./snapshot_file", "mem_backend": {"backend_path": "./base", "backend_type": "File"}, "track_dirty_pages": true, "resume_vm": false}, then PATCH /vm {"state": "Resumed"}.
8. Official doc: https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/snapshot-support.md