Add a Palette instanceof guard to BT.paletteSet so undefined/null inputs show a friendly canvas error instead of crashing on the next draw call. Rewrite five Palette error strings to plain language (slot-0 transparency rule, invalid size, invalid JSON shape, unknown color name). Rewrite the three Vector2i.div zero-divisor messages with the same pattern. Tests updated to cover the new guard and the rewritten message text. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com> Signed-off-by: Vaclav Vancura <commit@vancura.dev>