HermitCrab analysis: measure the required-slot look-ahead's repeated scans, narrowing and shadow diagnostics in steps #598

Four review points on the required-slot look-ahead (#575, which now also carries #576 and #579) are performance questions left unchanged until measured in steps:

• Repeated shape scans on the unordered path (RequiredSlotMaterial.AllPresent): each constraint rescans the shape, and constraints shared across slots are searched again. 575 F5, 576 F4
• Narrowing that may do unneeded work: an ordered slot above index 0 narrows even when no earlier check uses the result. 576 F3
• Shadow mode and RequiredSlotCheckObserver: kept for the soundness tests and step measurement; remove or keep once this investigation is done. 575 F4

Constraints: report the cost as steps (unifications, events) on Sena, Mbugwe, Aweti and Amharic, not wall time. Any shared scan or cache must be shown to stay within a few MB.

Done when each point has a step-count comparison and is either changed or closed with the numbers.