Add an integration response-header hook #782

Update, 7 October 2026. The hook is not in the module series (#1043 to #1094). An earlier version of the documentation set (#1047) added IntegrationResponseMutator, and it was taken out because no integration used it and the series' rule is against surface nothing uses. The hook returns with the first integration that needs one, and its design, 2026-07-30-integration-response-header-hook-design.md in #1084, is the starting bar for that work.

Description

Integrations can rewrite the HTML body today but cannot set headers on the outbound response. Add an IntegrationResponseMutator hook applied to the platform-neutral http::Response at the response convergence point, so an integration can add response headers, for example to request additional client evidence on a later request. This is a host-neutral extension point alongside the existing body-only HTML processing.

Done when

• An IntegrationResponseMutator trait and registry wiring (response_mutators, apply_response_headers).
• Each adapter applies registered response-header mutations on the outbound response.
• Covered by tests.

References

• Part of Pluggable, competition-neutral modules with a technical permission model #777. Not implemented by the module series, as the update above says.