http_resp.nx

http_resp.nx -- HTTP/1.1 response generator. Companion to http.nx (request parser). Assembles a valid HTTP response into a caller-mmapped output buffer: status line, headers, body. No internal allocation beyond tiny scratch. Response shape (RFC 7230 §3): HTTP-version SP status-code SP reason-phrase CRLF *header-field CRLF CRLF [ message-body ] Implementation notes: - Content-Length automatically emitted from body_len. Chunked transfer encoding is a future extension. - Connection: close emitted by default (no keep-alive yet). - Server: nishi emitted for sovereignty attribution. - Date header NOT emitted -- caller can pre-compute via timefmt.nx if clock sync matters for their use case. Invariants: HR1 Output buffer bounds-checked at every append; overrun returns a negative error rather than silent truncation. HR2 No internal state; each response is built from scratch. Thread-safe for the caller's own isolation concerns. HR3 Status codes + reason phrases kept in a fixed table; no arbitrary int -> string formatting on the response path. HR4 Content-Length header reflects body_len exactly; no mismatched framing that would confuse HTTP/1.1 clients.