shapes::polar_grid drew at least one radial divider (`radial.max(1)`), so radial 0, which
the command and the Polar Grid tool (Left arrow while dragging) accept, still drew a spoke.
Zero now draws none; the loop never divides by it.
The test checks 0, 1 and 2 radial dividers with no concentric ones give 1, 2 and 3 paths.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
# Conflicts:
# ROADMAP.md
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