Allow an A/A Experiment #1265

This was generated by AI from a grill-with-docs session.

Parent

#1252

What to build

An A/A Experiment: one commit and the same flags in both Arms, so that an Experiment measures how often the Arms disagree when nothing differs (grill Q2.1). Both Arms are built and run as they would be for any Experiment, and the result reaches a Verdict. If anything refuses identical Arms today, relax it for this case. docs/guide.md says how to run one, with the suites selected by --suite, and why.

Acceptance criteria

• An integration test runs an Experiment with one commit and the same flags in both Arms, against the fake Harness, through to a Verdict.
• docs/guide.md describes the A/A Experiment.