feat(phase10): decide virtio net refill notifications

[adybag14-cyber]

Summary

• add a bounded decideReceiveQueueRefillNotify() helper to drivers/net/virtio_net.zig so the existing refill reservation plan can be turned into one queue-local notify decision without entering descriptor writes or queue kicks
• extend zigux/tests/phase10_virtio_net.zig with focused coverage for empty-queue transitions, descriptor-threshold notifications, suppressed notifications, and zero-descriptor reservations
• refresh Documentation/zigux/phase10-virtio-net-slice.md so the lane note records the new notify-decision boundary

Why this lane

• the schedule prompt still names Phase 7, but current repo reality and prior P7-L14 work keep this lane narrowed to the existing Phase 10 virtio_net lab-driver packet
• the previous same-lane step on draft PR #10600 already reserved one refill pass against available receive-queue descriptors
• the next bounded, non-overlapping move in that same family was to consume that reservation plan for a queue-local notify decision without widening into live descriptor writes, DMA submission, queue kicks, receive completion, NAPI, or netdev lifecycle work

Validation

• zig fmt --check drivers/virtio/virtio.zig drivers/net/virtio_net.zig zigux/tests/phase10_virtio_net.zig zigux/tests/phase10_build.zig
• cd zigux/tests && zig build phase10-virtio-net-tests --build-file phase10_build.zig --summary all
  • result: Build Summary: 3/3 steps succeeded; 7/7 tests passed

Scope

• lane: P7-L14, narrowed to the existing Phase 10 virtio_net packet
• base 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, live kicks, NAPI, or receive-completion code

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