feat: use multiple threads when exporting snapshots

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Exporting a snapshot involves two CPU-bound tasks: Walking the graph, and encoding the ForestCAR.zst file. This PR moves each task to a separate thread, nearly doubling the throughput.

Benchmark (three runs each):

Command	Mean [s]	Min [s]	Max [s]	Relative
export calibnet (par-buffer)	27.646 ± 0.295	27.324	27.904	1.00
export calibnet (baseline)	48.516 ± 0.458	47.999	48.873	1.75

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[aatifsyed]

nice