FillExtents vertical copy distance was not clamped, allowing rows to be
placed overlapping each other when slide calculations returned large
values. Clamp to pairHeight + partSpacing minimum, matching FillLinear.
Also add FillStrategyRegistry.SetEnabled() to restrict which strategies
run — useful for isolating individual strategies during troubleshooting.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
AJ Isaacs
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