Document the on_turn callback contract

[avalyset]

on_turn had a type in each signature, a three-line note in the run_scenario_reps docstring, and nothing in the README. Which roles fire when, what turn_index and max_turns mean, and what happens on a failure could only be recovered by reading run_scenario and SingleTurnAuditor side by side. I mentioned this on #95.

What changes

OnTurn in model_auditor.py names the callback type and documents the contract as the code behaves today, measured on both runners:

• "auditor" only when the auditor wrote the probe, so not on a test_prompt turn
• "target" per turn
• "judge" once, at max_turns - 1, including when the judgment is a parse-failure ERROR
• nothing from a call that raised, and nothing after it
• no scenario name in the arguments, so calls interleave under max_workers > 1, and a retried rep in run_scenario_reps fires again

The nine on_turn annotations use the alias, the README parameter table gets an on_turn row, and the run_scenario_reps docstring points to the alias. OnTurn is not exported from simpleaudit/__init__.py.

No behaviour change: with docstrings stripped, the AST of the three changed modules differs only in the nine annotations, the alias and the imports that bring it in.

Testing

TestOnTurnContract in tests/test_single_turn.py runs ModelAuditor and SingleTurnAuditor and asserts the full sequence for each, including the target-down and judge-down paths. It passes on the unchanged code too, since it pins existing behaviour. Shifting the single-turn judge index from (0, 1, …) to (1, 1, …) fails it.

1349 passed, 19 skipped on Python 3.13.15, against 1348 on e5ec692 in the same environment.

Two edges, documented, not changed

• ModelAuditor(max_turns=0) runs no turns, fires (-1, 0, "judge") and judges an empty conversation as pass. The contract states max_turns is assumed to be at least 1.
• An async def callback is called but not awaited, so its body never runs. The contract says the callback must be a plain function.

Either could be its own PR if wanted.

A note from a trial merge

Merging main (9783293) into this branch gives no text conflicts, and the same holds for e5ec692 alone. With or without this PR, the merged tree fails test_single_turn.py::TestEntryPoints::test_run_sync_does_not_fall_back_to_the_turn_loop with TypeError: SingleTurnAuditor.run_async() got an unexpected keyword argument 'evidence_resolver', since main now passes evidence_resolver from run(). Worth knowing before the next sync.