feat(core): basic tensor operations (add, mul, matmul)

[Celz-Pch]

Goal

Implement element-wise and linear algebra primitives on Tensor.

Tasks

• Element-wise: +, -, *, / (with broadcasting, simple cases first)
• Reductions: sum, mean
• matmul for 2D tensors (naive triple-loop is fine to start)
• Operator overloading for clean syntax

Acceptance criteria

• Each op has a unit test comparing against hand-computed values
• API feels natural: auto z = x + y; and auto z = matmul(a, b);

Depends on

• #