490b8653

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.

Timeline