fix: contain mutation subprocesses

[darksectorai]

Summary

• run mutation compilation and tests in bounded worker pools
• propagate caller cancellation and CLI termination signals
• terminate Unix process groups so timed-out test descendants cannot survive
• cap captured output and set a configurable child Go memory target
• reject duplicate mutant IDs and use collision-free overlay directories

Root cause

exec.CommandContext terminated the immediate go test process on timeout, but a spawned test binary could remain alive and continue consuming resources. Mutation runs also created one goroutine per mutant, compilation had no timeout, and command output was buffered without a limit.

The command runner now places Unix children in a dedicated process group and kills the group on cancellation. The mutation timeout covers both compilation and testing, and cancellation propagates from the Cobra root context through file processing.

Child Go processes receive GOMEMLIMIT=2GiB by default. Set GOMU_CHILD_GOMEMLIMIT to override it.

Validation

• go test ./...
• make test
• make lint
• regression test proving a spawned descendant does not survive timeout
• regression tests for bounded output and memory-limit propagation

Portability

Unix platforms receive process-group termination. Other platforms retain direct-child termination behavior.