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.

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 members = [
     "crates/alknet-vault",
+    "crates/alknet-core",
 ]
 resolver = "2"