Prebuilt toolchain archives

Prebuilt pinned-toolchain archives (Arrow + Parquet + iceberg-cpp) consumed by scripts/fetch-deps.sh to skip the from-source bootstrap. Not a clink release: these are build-dependency artifacts, keyed by the version pins in scripts/versions.env and verified against the sha256 pins committed in scripts/deps-checksums.txt. Rebuilt via the deps-artifacts workflow (Linux) or scripts/package-deps.sh (macOS).