Hwaro::Services::NoCacheHandler

Dev responses must never be cached by the browser: watch rebuilds rewrite files whose Etag/Last-Modified derive from second-granularity mtimes, so two quick saves inside one second produce a "new" version whose validators match the cached one — the browser then 304s onto the stale copy until a hard refresh. no-store matches what other SSG dev servers ship. The header is set BEFORE call_next so it also reaches responses that flush their headers mid-body (static files larger than the response's 8KB output buffer). Skipped entirely when the user supplies their own Cache-Control via [serve.headers]/--header.