Implement CUDA Multipath Channel Simulation

This commit introduces the CUDA implementation of the multipath channel
model in `multipath_channel.cu`.

- Implements `multipath_channel_kernel` using shared memory for
  efficient, high-performance convolution.

- Provides a C-callable wrapper, `multipath_channel_cuda`, to
  orchestrate the full pipeline of data transfers and kernel
  launches.

- The wrapper includes support for multiple memory models (Explicit
  Copy, Unified Memory, ATS) via preprocessor directives to adapt to
  different build configurations.

- Adds a `interleave_channel_output_cuda` utility to convert signal
  data from the CPU's planar format to the GPU's interleaved format.

CMakeLists.txt

@@ -36,6 +36,23 @@ if(CUDA_ENABLE)
         set(CMAKE_CUDA_ARCHITECTURES 90)
     endif()
 
+    option(USE_UNIFIED_MEMORY "Enable CUDA Unified Memory path" OFF)
+    option(USE_ATS_MEMORY "Enable ATS Hybrid Memory path" ON)
+
+    if(USE_UNIFIED_MEMORY AND USE_ATS_MEMORY)
+        message(FATAL_ERROR "Cannot enable both USE_UNIFIED_MEMORY and USE_ATS_MEMORY at the same time.")
+    endif()
+
+    if(USE_UNIFIED_MEMORY)
+        add_compile_definitions(USE_UNIFIED_MEMORY)
+        message(STATUS "CUDA Unified Memory path enabled.")
+    elseif(USE_ATS_MEMORY)
+        add_compile_definitions(USE_ATS_MEMORY)
+        message(STATUS "CUDA ATS Hybrid Memory path enabled.")
+    else()
+        message(STATUS "CUDA Explicit Copy path enabled (default ATS was overridden).")
+    endif()
+
     add_compile_definitions(ENABLE_CUDA)
 endif()
 
@@ -297,6 +314,22 @@ if(GIT_FOUND)
   )
 endif()
 
+option(USE_UNIFIED_MEMORY "Enable CUDA Unified Memory path instead of explicit copies" OFF)
+
+option(USE_ATS_MEMORY "Enable Hybrid ATS (CPU->GPU) and Explicit Copy (GPU->CPU) path" OFF)
+
+if(USE_UNIFIED_MEMORY AND USE_ATS_MEMORY)
+  message(FATAL_ERROR "Cannot enable both USE_UNIFIED_MEMORY and USE_ATS_MEMORY at the same time.")
+endif()
+
+if(USE_UNIFIED_MEMORY)
+  add_compile_definitions(USE_UNIFIED_MEMORY)
+  message(STATUS "CUDA Unified Memory path enabled.")
+elseif(USE_ATS_MEMORY)
+  add_compile_definitions(USE_ATS_MEMORY)
+  message(STATUS "CUDA ATS Hybrid Memory path enabled.")
+endif()
+
 # Debug related options
 #########################################
 # asn1c skeletons have hardcoded this flag to make customized debug logs