Add design proposal: faster macOS VM standby/restore

[rgarcia]

Summary

Design proposal (RFC) for Faster macOS VM standby/restore (snapshot save/restore) under Apple's Virtualization.framework (the vz hypervisor) on Apple Silicon.

• Proposes compressing the vz machine-state.vzm file and overlapping that compression with the disk copy to cut standby latency for headless Linux guests on Apple silicon.
• Proposes wiring APFS clonefile(2) into lib/forkvm (currently a no-op stub off-Linux) so snapshot/fork disk copies on macOS become CoW-instant on same-volume APFS, with a sparse-copy fallback across volume boundaries.
• Adds a decompress-on-restore step for .vzm mirroring the existing memory/memory-ranges path, plus a shim control-API change so the disk copy can begin while the guest is still quiesced.
• Grounds every claim in concrete file:line refs for both hypeman and the Tart project (cited as prior art), and documents the hard constraint that vz's Save/Restore API is path-based and cannot be byte-streamed.
• Scoped to darwin/arm64 + macOS 14+, with a testing plan, platform edge cases (APFS volume boundaries, sparse-image desparsification), rollout milestones, and open questions.

Notes

• This PR adds a design document under docs/proposals/; it is documentation only, with no code changes.
• It references Tart, an open-source VM toolset built on the same Virtualization.framework, as prior art for design comparison.
• Opening as a draft to gather feedback on the approach before any implementation.

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