bd2e2a8d

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Mutation: control-group (no-op) mutations expected to survive

A control is a deliberate no-op: the plugin wraps an expression as
`(\ctrl -> ifMutation cmid ctrl ctrl) e` — the same expression on both
branches, a genuine non-diff — every `controlInterval` (12) real mutations,
with the counter primed so each module with an expression mutation gets at
least one. Both branches share the subtree via the lambda binder so the no-op
does not duplicate (and blow up) the instrumented expression.

A control changes no behaviour, so it is expected to survive. A killed control
means the mutation testing itself is unsound (a flaky/nondeterministic suite or
a harness bug), not a real kill, so it is reported separately as a warning and
excluded from the killed/survived score and the exit code.

Marked via the reserved operator name "Control" (no new record field).
New MutationOutcome variants OutcomeControlPassed/OutcomeControlFailed and
MutationResult variants MutationControlPassed/MutationControlFailed; the driver
remaps a control's exit-code classification accordingly. isMutationFailure is
False for both so a surviving control never trips fail-fast.

Regenerated the example manifest goldens (control entries only).

Timeline