Add standby / restore support for Mac vz hypervisor

[sjmiller609]

Summary

• teach the VZ shim about save/restore state, snapshot manifest handling, and platform configuration so standbys serialize RAM on macOS 14+ ARM
• extend the higher-level VZ hypervisor client and starter to expose Snapshot()/RestoreVM() plus runtime validation and manifest wiring
• document the new behavior and add an integration test that exercises the full VZ standby/restore cycle

Testing

• Added integration test

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Note

Medium Risk
Adds save/restore snapshotting to the vz hypervisor (stateful VM lifecycle and on-disk manifests), which can affect VM availability and data integrity if compatibility/validation or file handling is wrong. Scoped to macOS 14+ Apple Silicon and gated by capability checks, but still touches core VM lifecycle paths.

Overview
Enables standby/restore for the macOS vz hypervisor by adding a vz-shim PUT /api/v1/vm.snapshot endpoint that saves machine state to disk and writes a snapshot manifest (config.json) for later restore.

Adds restore boot flow to vz-shim (start vs restore based on RestoreMachineStatePath), persists platform identity via a stable generic machine identifier, disables the memory balloon device to improve save/restore compatibility, and extends VM state reporting to include save/restore-related states.

Wires this through lib/hypervisor/vz: Client.Snapshot() is implemented with a long-running HTTP client and Capabilities().SupportsSnapshot becomes arm64-only; Starter.RestoreVM() is implemented to read the manifest and start the shim in restore mode. Adds a darwin/arm64 integration test covering the full standby→restore cycle, updates docs/README wording, and makes Go test timeouts configurable via GO_TEST_TIMEOUT (set to 600s in CI).

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