VirtIO filesystem device (virtio-fs).

This implements the high-performance shared filesystem using virtiofs protocol for host-guest file sharing.

Architecture

Guest Driver (virtio-fs)
      │
      ▼ (virtqueue)
┌─────────────────────────┐
│      VirtioFs           │
│  ┌─────────────────┐   │
│  │   FuseSession   │   │
│  │  - INIT state   │   │
│  │  - features     │   │
│  └─────────────────┘   │
└───────────┬─────────────┘
            │ dispatch
            ▼
    FuseRequestHandler
      (implemented by arcbox-fs)

Module layout

• protocol: FUSE wire-protocol constants
• request: FuseRequest / FuseResponse envelopes
• session: FuseSession — INIT handshake + negotiated state
• handler: FuseRequestHandler trait
• device: FsConfig, VirtioFs, VirtioDevice impl