HV: add end-to-end Linux boot integration test #252

Problem

virt/arcbox-vmm/src/vmm/darwin_hv/mod.rs contains ~15 unit tests covering address math, PL011, fd duplication, and MMIO overlap — but zero of them boot a guest kernel. The existing 9/9 passing E2E suite recorded in internal-docs/status.md runs against the VZ backend.

There is no documented evidence in the repo or CI history that a Linux guest has booted end-to-end on HV with the full VirtIO device set mounted, networking reaching the host, and filesystem shares accessible.

Scope

Add at least one CI-runnable integration test that:

1. Boots a minimal Linux guest through initialize_darwin_hv() with VmBackend::Hv.
2. Confirms network reachability (ICMP to host gateway or HTTP fetch through vmnet bridge).
3. Confirms a VirtioFS share is mounted and at least one file readable.
4. Confirms vsock connectivity to the in-guest agent.
5. Cleanly shuts down (vCPU join, worker join, DAX munmap).

Not in scope (track separately if needed)

• Performance benchmarks (ABX-289 covers VirtioFS benchmarks).
• Rosetta x86_64 emulation path.
• Balloon device testing (blocked by sibling sub-issue).

Files

• tests/ — new HV integration test.
• tests/fixtures/ — minimal kernel + initramfs if not reusable from existing boot-assets.
• .github/workflows/ — wire into macOS CI runners.

Acceptance

• CI runs the test on every PR touching virt/arcbox-vmm/ or virt/arcbox-virtio*.
• Test completes in under 30s on an M-series runner.
• Failure modes produce actionable logs (vCPU state, virtqueue state, last MMIO access).