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

sunbeam-net/Cargo.toml

@@ -0,0 +1,38 @@
+[package]
+name = "sunbeam-net"
+version = "0.1.0"
+edition = "2024"
+description = "Pure Rust Headscale/Tailscale-compatible VPN client"
+license = "MIT"
+
+[dependencies]
+tokio = { version = "1", features = ["full"] }
+futures = "0.3"
+blake2 = "0.10"
+chacha20poly1305 = "0.10"
+hkdf = "0.12"
+hmac = "0.12"
+h2 = "0.4"
+http = "1"
+boringtun = "0.7"
+smoltcp = { version = "0.13", default-features = false, features = ["medium-ip", "proto-ipv4", "proto-ipv6", "socket-tcp", "std"] }
+x25519-dalek = { version = "2", features = ["static_secrets"] }
+crypto_box = "0.9"
+rand = "0.8"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+zstd = "0.13"
+bytes = "1"
+tokio-util = { version = "0.7", features = ["codec"] }
+base64 = "0.22"
+tracing = "0.1"
+thiserror = "2"
+ipnet = "2"
+
+[features]
+integration = []
+
+[dev-dependencies]
+tokio-test = "0.4"
+pretty_assertions = "1"
+tempfile = "3"