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/endpoint.rs

@@ -0,0 +1,5 @@
+//! Endpoint: `AlknetEndpoint`, `HandlerRegistry`, `EndpointError`.
+//!
+//! See `docs/architecture/crates/core/endpoint.md` for the full specification.
+
+// TODO: implement