feat(core): implement core types — ProtocolHandler, Connection, Capabilities (task: core/core-types)
Implement all core types in types.rs: ProtocolHandler trait (alpn + handle),
HandlerError (4 variants), Connection (quinn/iroh feature-gated enum dispatch,
OnceLock write-once identity, accept_bi/open_bi/close/remote_alpn/remote_addr),
BiStream trait, SendStream/RecvStream AsyncWrite/AsyncRead wrappers, StreamError,
From<StreamError> for HandlerError, Capabilities (Zeroize+ZeroizeOnDrop, immutable
builder API, Secret<String> wrapper, non-serializable), IdentityAlreadySet. Added
minimal Identity/AuthContext in auth.rs as foundation for the auth task.
Notable: quinn::Connection has no alpn() accessor so ALPN is stored separately
(from_quinn_with_alpn); iroh 0.35 types accessed via iroh::endpoint::*; iroh
Connection has no remote_address (returns None per spec).
Refs: docs/architecture/crates/core/core-types.md
Implements: ADR-002, ADR-007, ADR-014, ADR-022
glm-5.2
96938092ca