wiring-bats-testslisted

# Wiring bats Tests with Nix Lanes ## Overview Stand up integration tests under bats inside the nix sandbox: one derivation per `# bats file_tags=` directive, plus a `bats-default` that runs everything. Tests exercise real binaries via env-var injection from the flake, isolate per-test under `BATS_TEST_TMPDIR`, and use bats-island/bats-emo/bats-assert as the standard helper vocabulary. **Reference repos** (all under `~/eng/repos/`): - `tap` — fully on `amarbel-llc/bats`; cleanest separation (bats.nix as its own file). The **target shape** for new wirings. - `clown` — simplest stable shape: HTTP fixture with handshake-on-stdout - `madder` — most mature lane wiring (race + coverage + sftp/webdav lanes, version burnin), fully on `amarbel-llc/bats`. The worked example for the `extraBinaries` per-tag binary-fixture overlay ## When to Use **Bootstrap flow** — repo has no integration tests yet, or only ad-hoc shell scripts under `tests/`. Add `zz-tests_bats/`, wire `bats.nix`, expose `bats-default` as a flake check. **Migration flow** — repo has Go-level `*_test.go` exercising whole binaries via subprocess. Lift those into bats; keep narrow unit tests in Go. Tap is mid-migration; use it as the worked example. **Skip when:** - Pure library code with no CLI surface — Go's `testing` package is fine - One-off scripts in `playgrounds/` or `zz-explore/` - Tests must run on macOS without nix — bats works, but the lane builder assumes `amarbel-llc/nixpkgs` ## Migrating from a