Hwaro::Services::CustomHeadersHandler

Injects user-provided custom response headers on every dev-server response.

Runs after call_next so the configured headers always win over any headers set by built-in handlers (DevCorsHandler, CharsetHandler, 404 handler, redirect Location from IndexRewriteHandler, etc.). This gives predictable "what I put in [serve.headers] is what the browser receives" behaviour — exactly what users need when reproducing production server configuration locally.