feat: policy replay — measure a change before you ship it

[nayfly]

Every policy change this week was validated by hand: write a throwaway script, replay the trail, count what moved. That is the only question that matters — what does this change about decisions I have already seen? — and it caught something real each time: 51 hard_deny that were vocabulary rather than danger, two escalations from parameter names nobody could have guessed.

aetherya policy replay --candidate x.yaml --audit-path audit/decisions.jsonl

Two separations the report insists on:

• Loosened is not tightened. A decision that gets stricter is friction; one that gets looser is where a regression hides. Counted apart, and the exit code only reacts to adversarial regressions — gating on tightening would make every improvement fail the check meant to enable improvements.
• Traffic is not the corpus. Traffic measures friction, the corpus measures protection. A change that improves one while quietly breaking the other is the whole reason to replay.

The corpus is evaluated through the free-text path, not as structured calls. The first version got this wrong and reported nine regressions against an identical policy: several cases are only refused because of the parser and intent escalation, and ls / | xargs rm -rf carries no declared tool. A checker that fires on the policy it ships with is worse than no checker.

Replay reconstructs the shape of a recorded call, not the call: the trail stores parameter names and never their values, deliberately, so a credential cannot end up archived. raw_input still carries the command text the content rules read.

Verified in all three directions against the live trail — an identical candidate moves nothing, dropping read from the alias map shows two decisions tightening to hard_deny, and commenting out the rm -rf / family is caught as an adversarial regression with exit 1.

The test button on the breaker. It does not say the policy is right; it says what changed, and it works the same whether a human wrote the change or something proposed it.