The shared shard engine grows from the existing strict-output module
(runShardChild, killProcessGroup, signal forwarding, strict classifier).
scripts/test-strict-output.ts stays as a re-export so existing importers,
mock.module paths and the strict-output/run-shard-child tests are unchanged.
The engine inherits the global touchfile entry; the free runner's CLI-routing
fixture copies the new module.
