Both existing BatchMath tests used 2-3 particles while LaneCount >= 4, so
only the scalar tail ever ran, and both held y at 0 for every particle, so
every y-axis mutation was invisible. Add fixtures sized LaneCount * 3 + 1
with distinct nonzero x/y per particle, pinned against a scalar reference
computed independently in the test, plus explicit coverage of the early
return, negative deltaTime, length-mismatch, and empty-span guards on all
four BatchMath overloads.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
e3d52406db