feat(core): add axum HTTP router scaffold with auth middleware and stealth handoff

Add http feature flag with axum, hyper, hyper-util, tower, and http-body-util
dependencies. Create http module with auth middleware (extracts Bearer token,
calls IdentityProvider::resolve_from_token, attaches Identity to extensions)
and router scaffold (default 404 fallback, no operational routes yet). Replace
send_fake_nginx_404 with axum router handoff when http feature is enabled;
fake 404 behavior preserved when http is disabled. Wire HttpInterface with
build_router() method and pass IdentityProvider through Server to handle_connection.

crates/alknet-core/src/http/mod.rs

@@ -0,0 +1,5 @@
+pub mod auth;
+pub mod router;
+
+pub use auth::IdentityExt;
+pub use router::{build_router, serve_connection};