This commit adds a new standalone unit test, `test_multipath`, to
validate the CUDA implementation of the multipath channel.
The test performs the following steps:
- Generates a random input signal and channel model.
- Runs the CPU version (`multipath_channel_float`) to get a baseline result.
- Runs the CUDA version (`multipath_channel_cuda`) with the same inputs.
- Compares the two outputs by calculating the Mean Squared Error (MSE)
and ensures it is below a small tolerance.
