feat(net): add voltex_net crate with packet serialization

Introduces the voltex_net crate (no external dependencies) with a binary
Packet protocol over UDP. Supports 6 variants (Connect, Accept, Disconnect,
Ping, Pong, UserData) with a 4-byte header (type_id u8, payload_len u16 LE,
reserved u8) and per-variant payload encoding. Includes 7 unit tests covering
all roundtrips and invalid-type error handling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/voltex_net/src/lib.rs

@@ -0,0 +1,9 @@
+pub mod packet;
+pub mod socket;
+pub mod server;
+pub mod client;
+
+pub use packet::Packet;
+pub use socket::NetSocket;
+pub use server::{NetServer, ServerEvent, ClientInfo};
+pub use client::{NetClient, ClientEvent};