RemnantFiller: add placed parts as a single envelope obstacle instead
of individual bounding boxes to prevent the next drawing from filling
into inter-row gaps. Remove the topmost bounding-box part to create a
clean rectangular boundary.
PairsFillStrategy: guard against recursive invocation — remnant fills
within PairFiller create a new engine that runs the full pipeline,
which would invoke PairsFillStrategy again causing deep recursion.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
AJ Isaacs
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