test: add pytest suite (38 tests covering correctness, dtype, errors)

[asjad2401]

What this does

Adds a proper pytest suite replacing the previous ad-hoc test scripts.

Tests added (38 total)

• Dtype preservation — f32 in → f32 out, f64 in → f64 out for all functions
• Relu — correctness vs NumPy, 1D/2D/3D, shape, negative→zero
• Sigmoid — correctness vs NumPy, output range (0,1), boundary values (0→0.5, ±∞→0/1)
• Softmax — correctness vs NumPy, sums-to-one, axis=0/-1/middle, numerical stability
• Error handling — TypeError for int32/list inputs, ValueError for out-of-bounds axis

Running

maturin develop --release
pytest