docs(vz-backend): Phase 0 scope confirmation — FFI choice, coverage, pitfalls

Deliverable of Phase 0 in docs/vz-backend/PLAN.md. Desk-research only;
no production code. Adheres to the Linux-untouched gate.

The new PHASE_0_SCOPE.md answers all six gate-questions explicitly:

A. FFI strategy: objc2-virtualization 0.3.2 (madsmtm; full coverage of
   every Vz class the runtime needs, Zlib/Apache-2.0/MIT license,
   5340 dl/90d, part of the canonical objc2 ecosystem). Fallback:
   arcbox-vz 0.1.6.

B. Vz feature-coverage table. Every device class elastos-crosvm uses
   has a Vz equivalent. Two real deltas surfaced: Vz exposes the
   kernel console only as virtio-console (guest sees /dev/hvc0, not
   ttyS0); Vz vsock has no public CID API (host connects via the
   per-VM VZVirtioSocketDevice). Both accommodate via cfg-gated
   additive code — no Linux behaviour change.

C. Kernel-config audit. The shipped linux-amd64 vmlinux is 6.1.59
   with VIRTIO_CONSOLE; expected to work on Vz with two boot-arg
   adjustments. linux-arm64 uses a host-kernel-copy strategy and has
   no content-addressed artifact today — Phase 2 prototypes with a
   known-good Ubuntu cloud arm64 kernel; Phase 6 ships its own.

D. Tart + Lima study. 10 non-obvious pitfalls captured with
   file:line citations: disk caching mode (UTM #4840), persisted
   machine identifier, console=hvc0, multi-port virtio-console
   needs macOS 12+, no CID vsock API, initrd compression (LZ4 ok,
   zstd may fail), file-backed console rotation, vmnet NAT subnet
   conflict, async start + state delegate, Grand Central Dispatch
   queue per provider.

E. Risk register delta applied to PLAN.md: one Medium→Low reclassify
   (vsock semantic delta — now bounded), three new rows (arm64
   vmlinux artifact, initrd compression, vm_provider.rs AF_VSOCK
   adaptation).

F. Honest unknowns: CONFIG_VIRTIO_VSOCKETS in our 6.1.59 kernel
   (almost certainly yes — runtime uses AF_VSOCK today),
   6.1.59 + VZGenericPlatformConfiguration boot cleanliness,
   boot-latency target (target ≤ 3× Linux baseline).

G. Recommendation: Go. Proceed to Phase 1.

Appendix: 50-line feature-check binary deferred to Phase 1 day-1 per
the 10/10 prompt (requires the build environment Phase 1 sets up).

No code changes. No Linux behaviour change. PLAN.md update is a
risk-register delta only.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: SashaMIT