Validate VM ARP packets before reading addresses

[fallintoplace]

What changed

• route VM ARP validation through smoltcp::wire::ArpRepr::parse() before reading any address fields
• reject malformed ARP packets, wrong hardware/protocol families, and unknown ARP operations instead of assuming IPv4-sized protocol addresses
• add focused unit tests covering valid request/reply traffic plus malformed hardware type, protocol type, protocol length, and operation cases

Why

The VM ARP allow path used source_protocol_addr().try_into().unwrap(), so a guest could send an ARP packet with a non-IPv4 protocol address length and crash the proxy. ArpPacket::new_checked() only guarantees buffer bounds, not Ethernet/IPv4 ARP semantics.

Validation

• cargo fmt
• cargo test --no-run vm::tests
• /Users/hoangvu/Code/OSS/softnet/target/debug/deps/softnet-444df5915f996909 vm::tests