[msdmazvoid]

Summary

Phase 2 of the kouja storage backend, stacked on feat/kouja-storage-backend-phase1.

This adds the QEMU-side contract for remote disks: a prepared start plan, protocol-neutral disk argument construction, per-disk secret files, remote-source validation, and HMP run-state/resume support.

What changed

• Add qemu.StartPlan and StartWithPlan, while keeping Start for local-only VMs.
• Add buildDiskArgs for local and iSCSI attachments; NVMe/TCP and RBD return clear unsupported errors until their builders land.
• Add per-disk CHAP/PSK secret files under the now-0700 QEMU run directory, with cleanup on start failure, Stop, VM deletion, and stale-file sweep.
• Skip local device validation for kouja/ceph/remote-protocol disks.
• Add HMP info status, Paused, Resume, and ErrNotPaused.
• Add tests for iSCSI arguments, local arguments, missing attachments, secret lifecycle, stale cleanup, and remote validation.

Verification

• go test ./... -count=1
• go vet ./...
• gofmt clean

Chain

Base PR: #33 (feat/kouja-storage-backend-phase1). Merge in order #32 → #33 → this PR.

Review-artifact scratch files are intentionally excluded.