feat(core): implement core types (ProtocolHandler, Connection, Capabilities)

Implements the foundational types in alknet-core/src/types.rs per the
core-types architecture (ADRs 002, 007, 014, 022):

- ProtocolHandler trait (alpn + async handle) with HandlerError
- Connection wrapping quinn/iroh via internal enum dispatch (feature-gated);
  write-once identity via OnceLock, IdentityAlreadySet on second set
- SendStream/RecvStream concrete wrappers implementing AsyncWrite/AsyncRead
- BiStream convenience trait
- StreamError with canonical From<StreamError> for HandlerError
- Capabilities: non-serializable, Zeroize + ZeroizeOnDrop, immutable builder
  API (new/with_api_key/with_http_token/get) backed by a Secret<String> wrapper
- Minimal Identity and AuthContext in auth.rs as the foundation the auth task
  extends

13 unit tests cover Capabilities (build/get/clone/zeroize/redaction) and
Connection::set_identity (once succeeds, twice errors). Verified across
feature combos (default, no-default, iroh-only): build, clippy -D warnings,
test, fmt --check all clean.

(task: core/core-types)

crates/alknet-core/Cargo.toml

@@ -27,6 +27,6 @@ arc-swap = "1"
 async-trait = "0.1"
 tracing = "0.1"
 thiserror = "2"
-zeroize = { version = "1", features = ["derive"] }
+zeroize = { version = "1", features = ["alloc", "derive"] }
 bytes = "1"
 futures = "0.3"