Add gateway/error.rs with call_error_to_http_status,
call_error_to_http_status_with_identity, and call_error_to_http_response.
Maps the five protocol codes (NOT_FOUND/FORBIDDEN/INVALID_INPUT/TIMEOUT/
INTERNAL) to fixed HTTP statuses, splits FORBIDDEN into 401 (no identity) /
403 (identity present), maps HTTP_<status>-prefixed operation-level codes
to the status number (from_openapi fidelity), and defaults unknown
operation-level codes to 500. Retryable 503/429 errors carry a Retry-After
header when details.retry_after is present.
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.
