feat(phase10): reserve virtio net refill descriptors

[adybag14-cyber]

Summary

• add a bounded reserveReceiveQueueRefillDescriptors() helper to drivers/net/virtio_net.zig so one refill pass can be clamped against the descriptors currently available on the receive queue
• extend zigux/tests/phase10_virtio_net.zig with focused coverage for descriptor-budget exhaustion, exact-fit recycled-room reservations, and the zero-descriptors-per-buffer guard
• refresh Documentation/zigux/phase10-virtio-net-slice.md so the current lane note records the new reservation boundary

Why this lane

• the schedule prompt still says Phase 7, but the roadmap and prior P7-L14 history keep truthful packet-path work in the existing Phase 10 virtio_net lab-driver family
• the prior same-lane packet on draft PR #8032 already planned one refill batch from the saved mergeable-buffer plan
• the next bounded, non-overlapping move in that same family was to clamp that refill pass against available receive-queue descriptors without widening into live descriptor writes, DMA submission, kicks, receive completion, NAPI, or netdev lifecycle work

Validation

• /workspace/.toolchains/p7-l14/zig-x86_64-linux-0.17.0-dev.87+9b177a7d2/zig fmt --check /workspace/.scratch/p7-l14-refill-descriptor-budget/drivers/net/virtio_net.zig /workspace/.scratch/p7-l14-refill-descriptor-budget/drivers/virtio/virtio.zig /workspace/.scratch/p7-l14-refill-descriptor-budget/zigux/tests/phase10_virtio_net.zig /workspace/.scratch/p7-l14-refill-descriptor-budget/zigux/tests/phase10_build.zig
• cd /workspace/.scratch/p7-l14-refill-descriptor-budget/zigux/tests && /workspace/.toolchains/p7-l14/zig-x86_64-linux-0.17.0-dev.87+9b177a7d2/zig build test --build-file phase10_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 7/7 tests passed

Scope

• lane: P7-L14
• branch: zigux-builder/p7-l14-refill-descriptor-budget-20260525T1225Z
• files:
  • drivers/net/virtio_net.zig
  • zigux/tests/phase10_virtio_net.zig
  • Documentation/zigux/phase10-virtio-net-slice.md
• intentionally no edits to drivers/virtio/virtio.zig, shared Phase 10 build wiring, DMA paths, kicks, NAPI, or receive-completion code

Coordination

• stacked on draft PR #8032 because master still does not carry the bounded virtio_net packet this lane extends
• the next honest same-lane move is one queue-local publish or notify-decision helper that consumes the new reservation plan without entering live submission

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