Extract ConvergeFromAngle to deduplicate ~40 lines shared between
ConvergeStripeAngle and ConvergeStripeAngleShrink. Reduce BuildGrid
from 7 to 4 params and FillRemnant from 6 to 2 by reading context
fields directly. Remove unused angle parameter from FillRemnant.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>