Nightly mutants: baseline failure in `subduction_ephemeral`

The scheduled nightly mutation-testing run could not establish a baseline for `subduction_ephemeral`.

cargo-mutants exited with code 1 but reported zero missed mutants — this is the signature of a baseline-build failure (e.g. a feature flag the package doesn't declare, a missing dev-dep, or a flaky dependency).

- Run: https://github.com/inkandswitch/subduction/actions/runs/27860567943
- Date: 2026-06-20

Check the `Run cargo-mutants on subduction_ephemeral` step output and `mutants.out/log/baseline.log` in the artifact. Common fixes:
- Adjust the `features` entry for this crate in the workflow matrix.
- Update `.cargo/mutants.toml` if the issue is workspace-wide.

Start with the `Run cargo-mutants on subduction_ephemeral` step in the linked nightly run, then inspect `mutants.out/log/baseline.log` in its artifact. Compare the failure with the workflow matrix feature entry and `.cargo/mutants.toml`. Done means the crate establishes a baseline and the nightly mutation-testing job completes without the baseline-build failure.

Written by the indexing model from the issue text.