feat(core): initialize alknet-core crate with module skeleton

Create crates/alknet-core with Cargo.toml (dependencies, feature flags
quinn/iroh), src/lib.rs declaring types/auth/config/endpoint modules, and
skeleton files for each module with doc comments and TODO markers. Add the
crate to the workspace members list.

Both quinn (default-on) and iroh (opt-in) are optional and can be active
simultaneously per ADR-010. Dual license MIT OR Apache-2.0 inherited from
the workspace.

crates/alknet-core/src/lib.rs

@@ -0,0 +1,12 @@
+//! alknet-core: Core library for ALPN-based protocol dispatch.
+//!
+//! Every handler crate depends on this crate. It provides the
+//! [`ProtocolHandler`][crate::types::ProtocolHandler] trait, the
+//! [`Connection`][crate::types::Connection] wrapper, auth primitives,
+//! hot-reloadable configuration, and the [`AlknetEndpoint`][crate::endpoint::AlknetEndpoint]
+//! that dispatches incoming QUIC connections by ALPN string.
+
+pub mod auth;
+pub mod config;
+pub mod endpoint;
+pub mod types;