[codex] add linux amd64 startup snapshots

[Vbitz]

Summary

Adds startup snapshot capture and restore support for the linux/amd64 KVM backend.

This includes:

• capturing sparse guest memory plus KVM vCPU state, MSRs, FPU/LAPIC/XSAVE/XCRS, IRQ chip, PIT, clock, and virtio MMIO device state
• restoring managed linux/amd64 sessions from a captured snapshot
• wiring SnapshotDir and RestoreSnapshot through the linux/amd64 runtime backend
• triggering snapshot capture from guest init through the configured snapshot MMIO page
• making KVM managed run loops interruptible so session cleanup does not hang on cancellation
• adding an integration test that captures a persistent Linux runtime snapshot, restores it, and executes a command

Closes #31.

Notes

Startup snapshots currently support one vCPU on linux/amd64. Multi-vCPU snapshot startup is rejected explicitly.

Validation

go test ./internal/hv/kvm ./internal/vm ./internal/cmd/init ./internal/vmruntime ./internal/virtio ./internal/managed/...

A local timing smoke check showed restore-to-ready around 35-43ms versus cold persistent Linux ready around 371-387ms with 256MB/1 CPU on this machine.