kernels (K3): cell kinds in core/, capability guard first, self-contained block collision

- core/cell_kind.cuh holds KIND_ACTIVE/INNER/OUTER (was a constexpr in
  blocks.cu that step/block_channel.cu and the channel_root.cpp shim
  relied on or redefined); blocks.cu, block_channel.cu and the shim now
  include it.
- blocks.cu checks the lattice/model #error guard before the
  BLOCK_ROOT_BC guard and mode includes, so an unsupported build reports
  the real reason first.
- operators/block_collision.cuh includes what it uses (params, macro,
  SRT/MRT collision) instead of relying on blocks.cu's include order.
Preprocessor-only: same kernels, same statements, same arithmetic.

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

tests/unit/cpp/channel_root.cpp

@@ -41,7 +41,7 @@ inline float __fdiv_rn(float a,float b){return a/b;}
 #include "boundary/inlet_outlet.cuh"
 #include "step/one_step_double.cu"
 #else
-constexpr unsigned char KIND_OUTER=2;
+#include "core/cell_kind.cuh"
 #include "step/block_channel.cu"
 #endif
 int main(){