Add QEMU-driven integration test harness

[bketelsen]

Summary

• Add a shell-based QEMU integration test harness modeled after snosi's test/ infrastructure
• Validates full install-to-boot, /etc overlay persistence, A/B partition layout, and A/B update flows
• No changes to nbc source code — testing enablement only
• Bump Go version from 1.24 to 1.26 in go.mod and CI workflows

New files

File	Purpose
tests/lib/vm.sh	QEMU lifecycle: OVMF discovery, VM start/stop/cleanup
tests/lib/ssh.sh	SSH: keypair gen, vm_ssh, vm_scp, wait_for_ssh
tests/nbc-qemu-test.sh	Orchestrator: build → install → inject SSH → boot → run tests
tests/tests/01-boot.sh	Guest: boot health, mounts, kernel cmdline
tests/tests/02-etc-overlay.sh	Guest: /etc overlay mount, write persistence
tests/tests/03-ab-layout.sh	Guest: 4 partitions, filesystem types, config.json, slot A active
tests/tests/04-update.sh	Guest: nbc update, inactive root populated, config persists
.github/workflows/test-qemu.yml	CI workflow (manual dispatch)

Usage

make test-qemu IMAGE=ghcr.io/frostyard/snow:latest

Requires root, KVM, and OVMF firmware.

Test plan

• Run make test-qemu IMAGE=ghcr.io/frostyard/snow:latest locally with KVM
• Trigger workflow_dispatch on GitHub Actions to verify CI workflow
• Verify all 4 test tiers pass (boot, /etc overlay, A/B layout, update)

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