`buffer.memset` creates `len` bytes filled with `value & 0xff`.
Reference CPU implementation:
```rust
let out = vec![(value & 0xff) as u8; len as usize];
```
WGSL spelling note: each lane writes the same masked byte value. This op uses
the `decode-bytes-to-bytes` archetype as a temporary fallback because the
locked vocabulary has no buffer-fill archetype yet.