Give a zero remainder the sign of the divisor

[krrishapatel]

Proposed changes

Fixes #4315.

mx.remainder gives a zero result the sign of whichever code path produced it. All nine inputs here are -0.0 % 3.0:

np.signbit(np.array(mx.remainder(mx.full((9,), -0.0), mx.full((9,), 3.0)))).astype(int)
# [0 0 0 0 0 0 0 0 1]

Lane 8 is the scalar residual. A strided view disagrees with a contiguous array the same way, and neither matches numpy, which gives a zero remainder the sign of the divisor.

The floored fixup skips zero:

if (r != 0 && (r < 0 != b < 0)) {
  r += b;
}

so the zero keeps whatever sign it had, and asd::remainder and std::remainder do not agree on that. The guard cannot just be dropped, since adding b to a zero returns b itself, so the zero case sets the sign directly. Integers are untouched, they have no signed zero. Non-zero results are unchanged.

The same guard was in all four backends, so all four are fixed. CPU is verified locally on an M-series Mac. Metal and CUDA are the same edit to the same guard but I cannot run them here, so please treat those two as unverified.

test_remainder_signed_zero covers the residual lane, the strided view, both divisor signs against numpy, the % operator, and that integers are unaffected. It fails on main at the first assertion.

No overlap with #4108. That one floors the quotient for divmod and integer floor_divide; this is the sign of a zero remainder. They share two files but different structs, FloorDivide versus Remainder. I merged #4108 into this branch to check: no conflicts, both fixes active, test_ops.py green with both test sets.

Checklist

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• I have added tests that prove my fix is effective or that my feature works
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