feat(http): initialize alknet-http crate with module skeleton

Add crates/alknet-http with Cargo.toml, src/lib.rs, and the five
subsystem modules (server, gateway, client, adapters, websocket) per
ADR-039 (server + client host colocated). The mcp feature gate pulls in
rmcp with streamable HTTP transport features only (ADR-037 — no stdio);
h3/WebTransport is absent (deferred per ADR-044). alknet-core and
alknet-call use workspace path deps. The crate is added to the workspace
members list.

Cargo.toml

@@ -3,6 +3,7 @@ members = [
     "crates/alknet-vault",
     "crates/alknet-core",
     "crates/alknet-call",
+    "crates/alknet-http",
 ]
 resolver = "2"