Replace bits-and-blooms/bitset with roaring bitmaps

[ValentaTomas]

Replace all direct usage of bits-and-blooms/bitset with roaring/v2 (v2.17.0). Uses the new Ranges() iterator (#522) for range iteration in CreateMapping and BitmapRanges.

[cursor]

PR Summary

Medium Risk
Touches snapshot diff metadata and dirty-range iteration used for exporting memory/rootfs, so subtle bitmap semantic differences (cardinality, range boundaries, uint32 limits) could corrupt diffs or mappings if incorrect. No auth/security changes, but it impacts core snapshotting and I/O paths.

Overview
Replaces bits-and-blooms/bitset usage with roaring/v2 bitmaps across snapshot diff generation and export paths (block cache dirty tracking, range iteration, Firecracker memory metadata decoding, and header mapping construction), including switching range iteration to Bitmap.Ranges() and introducing a thread-safe syncroaring.Bitset wrapper for cache dirty ranges; updates related tests and diff-metadata builder APIs accordingly.

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[levb]

a few nits, otherwise LGTM

[jakubno]

Nice! 🚀 1 suggestion left in comments

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[levb]

LGTM