By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Keep mutation packages off the forward-compatibility build horizon-advance does not ship list-t or stm-containers, so adding them to sydtest's dep tree broke 'nix flake check' on the forwardCompatibility output. Revert the MutationMode runner back to a TVar (sydtest itself no longer needs stm-containers) and route forwardCompatibility through a new sydtestReleaseWithoutMutation that excludes every sydtest-mutation-* package. The plugin still uses stm-containers internally — it is a GHC-version-pinned plugin and does not build on horizon anyway. Also inline the two defaults that used to live as fields on bottom- seeded record values (defaultMutationFailFast, defaultCoverageParentRetry) so callers do not accidentally evaluate the strict 'error' fields.