feat(net-core): composition primitives

[NotAProfDev]

Closes #12

Summary

Implements the zero-runtime composition primitives for oath-net-core — the foundation every other network crate builds on.

Changes

Cargo.toml (root)

Added five new workspace dependencies required by net-core:
bytes, http, http-body, futures-core, futures-sink

crates/net/core/Cargo.toml

Wired the five workspace deps. No tokio, hyper, serde, or thiserror — the zero-runtime constraint is preserved.

crates/net/core/src/service.rs (new)

• Service<Req> trait — RPITIT return type, &self receiver, no poll_ready
• Layer<S> trait — wraps one service into another
• ServiceBuilder<L> — compile-time compositor; first .layer() call is permanently outermost; derives Clone; layer() is #[must_use]
• Identity — no-op layer; termination case for the Stack recursion
• Stack<Inner, Outer> — composes two layers; Inner applied first (closer to leaf), Outer executes first on requests

crates/net/core/src/error_kind.rs (new)

• ErrorKind — coarse middleware routing enum (Timeout, Connection, Throttled, Auth, Client, Server, Unknown); #[non_exhaustive]
• HasErrorKind — implemented by backend error types so middleware layers can branch without inspecting concrete types

crates/net/core/src/lib.rs

Updated with module declarations and pub use re-exports for all public types.

Design decisions

• No Send + Sync supertrait on Service — kept general so test doubles, single-threaded adapters, and WASM targets can implement the trait; call sites that require thread-safety add their own bounds
• No async fn in traits — RPITIT (fn → impl Future + Send) avoids async-trait, boxing, and per-call allocation
• No dyn anywhere — entire stack resolves to a single monomorphized type at compile time; zero virtual dispatch on the hot path