Nightly mutants: surviving mutants in `subduction_iroh`

The scheduled nightly mutation-testing run found surviving mutants in `subduction_iroh`.

- Run: https://github.com/inkandswitch/subduction/actions/runs/26323500212
- Date: 2026-05-23

Download the `mutants-out-subduction_iroh` artifact from the run and inspect `outcomes.json` / `missed.txt` for the list.

Action: add tests that kill the surviving mutants, or annotate them as out-of-scope in `.cargo/mutants.toml`.

Download the mutants-out-subduction_iroh artifact from the linked GitHub Actions run and inspect outcomes.json and missed.txt to identify the surviving mutants. Read the affected subduction_iroh code and its existing tests, then add tests that kill the mutants or record justified exceptions in .cargo/mutants.toml. Done means no in-scope mutants remain surviving in the nightly run.

Written by the indexing model from the issue text.