[op] _to_copy / to.dtype

Parent: #15

## Op / kernel
**_to_copy / to.dtype**

Implement dtype conversion on-device (float64/float16/bfloat16/int).

## Acceptance criteria
- [ ] ATen / custom op registered on `PrivateUse1`
- [ ] Host (CPU sim) implementation
- [ ] OpenCL kernel + runtime launch (or explicit deferral note)
- [ ] Test vs CPU reference in `tests/`
- [ ] Linked PR closes this issue

## Category
`cat-dtype`
