Enable unused return value checker

`-Xreturn-value-checker=full`

Projects:
* [ ] `JakeWharton/cite`
* [ ] `JakeWharton/confundus`
* [ ] `JakeWharton/crossword`
* [ ] `JakeWharton/dalvik-dx`
* [ ] `JakeWharton/dependency-tree-diff`
* [ ] `JakeWharton/dependency-watch`
* [ ] `JakeWharton/diffuse`
* [ ] `JakeWharton/docker-mbsync`
* [ ] `JakeWharton/dumbo`
* [ ] `JakeWharton/finalization-hook`
* [ ] `JakeWharton/gitout`
* [ ] `JakeWharton/hardcover-data-sync`
* [ ] `JakeWharton/jakewharton.com`
* [ ] `JakeWharton/kmp-missing-targets`
* [ ] `JakeWharton/mosaic`
* [ ] `JakeWharton/picnic`
* [ ] `JakeWharton/plex-auto-trash`
* [ ] `JakeWharton/plex-orphaned-files`
* [ ] `JakeWharton/radarr-folder-fixer`
* [ ] `JakeWharton/singular-solution`
* [ ] `JakeWharton/test-distribution-gradle-plugin`
* [ ] `JakeWharton/timber`
* [ ] `JakeWharton/video-swatch`
* [ ] `cashapp/licensee`
* [ ] `cashapp/molecule`
* [ ] `cashapp/paraphrase`
* [ ] `cashapp/turbine`
* [ ] `lysine-dev/okio`
* https://github.com/lysine-dev/okio/pull/1733
* [ ] `lysine-dev/okhttp`
* [ ] `lysine-dev/retrofit`
* [ ] `SQLDelight/Grammar-Kit-Composer`
* [ ] `SQLDelight/sql-psi`
* [ ] `SQLDelight/SQLDelight`
* [ ] `square/kotlinpoet`
* [ ] `square/zstd-kmp`

Start with the `-Xreturn-value-checker=full` option and the project checklist in this issue. Review each listed repository, noting whether the option can be enabled and whether the build remains successful; the linked `lysine-dev/okio` pull request shows that work is already underway there. Done means the applicable projects have been checked off.

Written by the indexing model from the issue text.