feat: add inverse trigonometric functions (asin, acos, atan)

[AndrewTKent]

Summary

Adds asin, acos, and atan with full autodiff support across all backends.

Changes

Layer	Files
Core API	tensor.rs, op.rs
Autodiff	backprop.rs
CPU	mkl.rs, accelerate.rs
CUDA	cuda_utils.cuh, unary.cu
Metal	unary.metal, unary.rs, metal_backend/mod.rs
Tests	tensor_tests.rs, grad_tests.rs

Backend Coverage

Backend	f32	f64	f16	bf16	fp8
CPU	✓	✓	✓	✓	✓
CUDA	✓	✓	✓	✓	✓
Metal	✓	—	✓	✓	—
MKL	✓	✓	—	—	—
Accelerate	✓	✓	—	—	—

Gradients

d/dx asin(x) = 1/√(1-x²)
d/dx acos(x) = -1/√(1-x²)  
d/dx atan(x) = 1/(1+x²)

Test values validated against PyTorch.

Usage

let t = Tensor::new(&[0.0f32, 0.5, -0.5], &Device::Cpu)?;
let asin = t.asin()?;  // [0.0, 0.5236, -0.5236]
let acos = t.acos()?;  // [1.5708, 1.0472, 2.0944]
let atan = t.atan()?;  // [0.0, 0.4636, -0.4636]

Closes #3270