kernels (K4): withdraw the address helpers; keep only fill_invalid

Horsea's gates showed block_ddf/block_slot change real PTX in
BlockHaloCopy, BlockExplode and BlockExplodeProjected (6/10 modules;
BlockExplodeProjected 38 -> 40 registers) although the results stayed
exact. The stack's bar is byte-identical PTX for naming-only steps, so
those three kernels return to their K3 expressions, the helpers leave
core/layout.cuh, and test_projected_explosion goes back to its K3 form.
fill_invalid stays (its kernels were byte-identical). K4 vs K3 is now
only the NaN-fill helper.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/lbm/kernels/blocks.cu

@@ -167,7 +167,7 @@ extern "C" __global__ void BlockStep(const fpxx* in, fpxx* out, unsigned int hal
     const unsigned char cell_kind = kind != nullptr ? kind[cell] : KIND_ACTIVE;
     float f[NQ];
     if (cell_kind == KIND_OUTER || (flags != nullptr && is_solid(flags[cell]))) {
-        for (int q = 0; q < NQ; ++q) f[q] = NAN;
+        fill_invalid(f);
     } else {
         unsigned long j[NQ];
         compute_neighbors(x, y, j);  // Core neighbors stay inside the window: no wrap.

src/CelerisLab/lbm/kernels/core/cell_kind.cuh

@@ -7,4 +7,10 @@
 
 constexpr unsigned char KIND_ACTIVE = 0u, KIND_INNER = 1u, KIND_OUTER = 2u;
 
+// NaN populations mark a cell without valid state (OUTER ghost or solid core).
+__device__ __forceinline__ void fill_invalid(float* f)
+{
+    for (int q = 0; q < NQ; ++q) f[q] = NAN;
+}
+
 #endif

src/CelerisLab/lbm/kernels/refinement.cu

@@ -9,6 +9,7 @@
 #include "core/params.cuh"
 #include "core/flags.cuh"
 #include "core/obs.cuh"
+#include "core/cell_kind.cuh"
 #include "operators/macro.cuh"
 #include "operators/collision_srt.cuh"
 #if COLLISION_MODEL == 2
@@ -71,7 +72,7 @@ extern "C" __global__ void PatchFineStep(const float* in, float* out, const uint
     // that held explosion data. Inner halo only streams; it never collides.
     if (x < 2 || y < 2 || x >= NX - 2 || y >= NY - 2
             || (flags != nullptr && is_solid(flags[k]))) {
-        for (int q = 0; q < NQ; ++q) f[q] = NAN;
+        fill_invalid(f);
     } else {
         unsigned long j[NQ];
         compute_neighbors(x, y, j);  // These interior targets never wrap.

src/CelerisLab/lbm/kernels/step/block_channel.cu

@@ -26,7 +26,7 @@ extern "C" __global__ void BlockChannelStep(
     const unsigned long cell = (unsigned long)blockIdx.z * TOTAL_CELLS + k;
     float f[NQ];
     if (kind != nullptr && kind[cell] == KIND_OUTER) {
-        for (int q = 0; q < NQ; ++q) f[q] = NAN;
+        fill_invalid(f);
     } else {
         unsigned long j[NQ];
         compute_neighbors(x, y, j);