feat(core): implement Tensor class (n-dimensional array)

[Celz-Pch]

Goal

Provide the foundational data structure for all computation.

Tasks

• Tensor<T> template class supporting float/double
• Shape, strides, contiguous storage
• Basic constructors: zeros, ones, from data, random
• Indexing and slicing
• Pretty printing for debugging

Acceptance criteria

• Unit tests cover construction, indexing, and shape manipulation
• Memory is correctly managed (no leaks under valgrind/asan)

References

• NumPy ndarray semantics
• PyTorch ATen Tensor