`buffer.memcpy` copies a byte buffer unchanged. It has the identity law:
copying the output again produces the same bytes.
Reference CPU implementation:
```rust
let out = input.to_vec();
```
WGSL spelling note: each lane copies `input[idx] & 0xffu` to `out[idx]`. This
op uses the `decode-bytes-to-bytes` archetype as a temporary fallback because
the locked vocabulary has no buffer-copy archetype yet.