A fresh worktree needed a cold 'make dev-setup' (meson configure + ~24 CPU-minutes of GStreamer/FFmpeg/glib compilation plus subproject clones) before it could run a single cgo Go test. But the libraries the tests link against are a pure function of the meson inputs (meson.build, subprojects/*.wrap, rust/, Cargo.lock), not of the branch's Go code, and the dev build installs them into the build dir itself. hack/seed-build-dir.sh (make dev-setup-fast) copies lib/include/libexec/ bin from an already-built checkout in under a second, retargets the .pc prefix paths, and stubs js/app/dist + the lexicon stamp. It refuses to seed when the tracked native build inputs differ from the donor's, and a post-seed canary (bin/gio against the copied libs) catches donors built against a different libc — notably a host-built build dir seeded inside the builder container. hack/test-env.sh wraps any command with the PKG_CONFIG_PATH / LD_LIBRARY_PATH / CGO_LDFLAGS / GST_PLUGIN_PATH the cgo tests need, or prints export lines when run without arguments. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>