Fix position-swap loop for charge exchange across all geometries

The previous `for (int idim = 0; idim < PIdx::w; ++idim)` loop assumed
position attributes were stored at indices 0..PIdx::w-1, which is true
for 1D_Z, XZ, and 3D, but NOT for RZ, RCYLINDER, and RSPHERE — there
theta and phi come after `w` in PIdx. Replace with a range-based for
over a `static constexpr std::array<int, N>` whose contents are
selected per-dimension via `#if defined(WARPX_DIM_*)`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: RemiLehe

Source/Particles/Collision/BinaryCollision/DSMC/SplitAndScatterFunc.H

@@ -17,6 +17,26 @@
 #include "Utils/ParticleUtils.H"
 #include "Utils/WarpXAlgorithmSelection.H"
 
+#include <array>
+
+// Indices in PIdx of the spatial-position attributes, for the current
+// dimensionality. Note that they are not contiguous in PIdx for RZ-family
+// geometries (theta/phi come after `w`), so a simple 0..PIdx::w-1 loop would
+// miss them.
+#if defined(WARPX_DIM_1D_Z)
+    static constexpr std::array<int, 1> position_indices = {PIdx::z};
+#elif defined(WARPX_DIM_XZ)
+    static constexpr std::array<int, 2> position_indices = {PIdx::x, PIdx::z};
+#elif defined(WARPX_DIM_RZ)
+    static constexpr std::array<int, 3> position_indices = {PIdx::x, PIdx::z, PIdx::theta};
+#elif defined(WARPX_DIM_RCYLINDER)
+    static constexpr std::array<int, 2> position_indices = {PIdx::x, PIdx::theta};
+#elif defined(WARPX_DIM_RSPHERE)
+    static constexpr std::array<int, 3> position_indices = {PIdx::x, PIdx::theta, PIdx::phi};
+#elif defined(WARPX_DIM_3D)
+    static constexpr std::array<int, 3> position_indices = {PIdx::x, PIdx::y, PIdx::z};
+#endif
+
 /**
  * \brief This class defines an operator to create product particles from DSMC
  * collisions and sets the particle properties (position, momentum, weight).
@@ -360,9 +380,8 @@ public:
                 // For charge exchange, swap positions at which the products are created
                 // to ensure local charge conservation. For instance in the reaction
                 // `A+ + B -> A + B+`, B+ will be created at the position of A+, instead of that of B.
-                // Position components occupy indices 0 through PIdx::w-1 in m_rdata.
                 if (mask[i] == int(ScatteringProcessType::CHARGE_EXCHANGE)) {
-                    for (int idim = 0; idim < PIdx::w; ++idim) {
+                    for (int idim : position_indices) {
                         soa_products_data[2].m_rdata[idim][slot2_idx] = soa_1.m_rdata[idim][src1_idx];
                         soa_products_data[3].m_rdata[idim][slot3_idx] = soa_0.m_rdata[idim][src0_idx];
                     }