feat(net): scaffold sunbeam-net crate with foundations

Add the workspace crate that will host a pure Rust Headscale/Tailscale-
compatible VPN client. This first commit lands the crate skeleton plus
the leaf modules that the rest of the stack builds on:

- error: thiserror Error enum + Result alias
- config: VpnConfig
- keys: Curve25519 node/disco/wg key types with on-disk persistence
- proto/types: PascalCase serde wire types matching Tailscale's JSON

Cargo.toml

@@ -11,7 +11,7 @@ name = "sunbeam"
 path = "src/main.rs"
 
 [workspace]
-members = ["sunbeam-sdk"]
+members = ["sunbeam-sdk", "sunbeam-net"]
 resolver = "3"
 
 [dependencies]