add a fail-fast flag

[CGamesPlay]

This resolves #41. Full disclosure: this change was coded by an AI agent. Original prompt:

Please build a feature for fast failures. It should be a test case configuration option (similar to "detached") named "fail_fast". When set, if the test case fails for any reason (exit status, snapshot, etc), the entire test document immediately stops.

I reviewed the code and tests, and tested the result in my actual scrut deployment.

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