Soul's pass on 41440cb found BatchMath.cs:115
(AddRadialFalloffAcceleration2D(ReadOnlySpan<float2>, ...)) accumulates
onto the caller's acceleration buffer, but no test ever seeded that
buffer with nonzero values: mutating += to = passed all 133 tests. The
float-array overload already had this guard
(RadialFalloffAccumulatesOntoExistingAcceleration); add the float2
equivalent, with a nonzero, per-element-distinct starting buffer, mixing
in-range and out-of-range particles so both the accumulated and
unchanged-seed cases are checked.
Checked every other += site in BatchMath.cs for the same gap: the
array-overload's scalar tail and vector body, and both
IntegrateSemiImplicitEuler2D overloads, all already seed their
accumulator buffers (vx/vy/px/py, velocity/position) with nonzero,
per-element values in their existing tests. Only this one site was bare.
Verified by hand-mutating BatchMath.cs:115 (+= to =): the new test fails
(expected 6.78, actual 0.48); reverted the source byte-exactly after.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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