Rule-testing framework for detector authors

[Gbangbolaoluwagbemiga]

Epic: Custom Rules & Configuration

Summary

Provide a harness so rule authors can write fixture-based tests + snapshots easily.

Background & Problem

Lowering the testing barrier improves detector quality + contributions.

Acceptance Criteria

• Authors can add a rule + tests with minimal boilerplate.
• Snapshot workflow documented.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.