V3 deterministic PDF merge capability pack. By-value connector composition (service_kind=cli, no separate ingredient): the local Poppler pdfunite CLI exposes readiness, exact two-PDF merge, and bounded multi-PDF merge operations. The executor materializes ordered file_ref arrays into engine-managed temp paths, expands them into discrete argv elements in caller order, and owns the output path captured as result.file_ref. The pack exposes no arbitrary pdfunite flags, encrypted-PDF password handling, caller-controlled output path, stdin mode, shell wrapper, or network egress. Requires pdfunite on PATH (Debian/Ubuntu: apt install poppler-utils; macOS: brew install poppler).