Add coefficient-wise modulus operator (%) for Array

Summary

• Add scalar_modulus_op functor in BinaryFunctors.h
• Add operator% for array-array and array-scalar forms in ArrayCwiseBinaryOps.inc
• Integer types only (enforced by compiler since % is undefined for floats)
• No SIMD vectorization (integer modulus has no standard packet op)

Example usage:

Array3i a(7, 8, 9), b(2, 3, 4);
Array3i c = a % b;   // [1, 2, 1]
Array3i d = a % 2;   // [1, 0, 1]

Test plan

• Array % Array works correctly
• Array % scalar works correctly
• 2D ArrayXXi % scalar works correctly
• Compound expressions (a + b) % c work correctly
• Existing array_cwise test suite passes

Closes #2837