The `modulo` filter used JavaScript's `%` (truncated remainder, sign
follows the dividend). Shopify/Ruby Liquid uses floored modulo, where the
result takes the sign of the divisor. Since liquidjs advertises Shopify
compatibility, negative operands produced the wrong sign.
Use `((v % arg) + arg) % arg` to match Ruby's `%`. Positive-operand
results are unchanged.
