refactor(xpp/http): move static response constructors to Response

[mivinci]

Summary

ResponseBuilder::ok/created/no_content/bad_request/not_found/internal_server_error were Response factory constructors in disguise — they build through a throwaway builder, never touching builder state. Response's doc comment already promised this API (Response r = Response::ok("hello");) but the methods lived on the builder.

Moves them to Response (out-of-line definitions, going through Response::builder() exactly as before), leaving ResponseBuilder a pure fluent configurator/terminator. This matches hyper's split: value-type constructors vs Response::builder() for the verbose path.

Also adds Body overloads (Response::ok(Body), etc.) — the terminator already accepts Body, and streaming handlers no longer need the builder chain just to set a 200 status:

// before
return ResponseBuilder().status(StatusCode::Ok).body(Body::from_channel(std::move(rx)));
// after
return Response::ok(Body::from_channel(std::move(rx)));

Changes

• libxpp/xpp/http/response.h — statics moved (+4 Body overloads), ResponseBuilder back to pure builder
• libxpp/xpp/http/response_test.cpp, server_test.cpp — 30 mechanical call-site replacements + simplified streaming constructions
• docs/libxpp/http/{README,body,server}.md — call sites updated, streaming examples simplified

No behavior change.

Validation

• http_response_test 17/17, http_server_test 13/13
• Full ASan suite 80/80 (xdns_test network flake passes standalone)
• mdBook builds clean