Implement the ALPN router and endpoint in endpoint.rs: AlknetEndpoint with
quinn+iroh accept loops (both feature-gated, both Option), HandlerRegistry
(new/register/get/alpn_strings with panic-on-duplicate), dispatch via
tokio::spawn by ALPN, AuthContext construction from connection
(alpn/remote_addr/fingerprint/identity), TLS identity modes (RawKey RFC 7250
via on-the-fly cert resolver, X509 from files, SelfSigned via rcgen),
EndpointError enum, graceful shutdown with drain timeout + force close.
ACME deferred as TODO per task spec. 55 tests (--all-features), 52 (default),
47 (no-default); clippy clean across all 3 feature combos.
Refs: docs/architecture/crates/core/endpoint.md
Implements: ADR-010