subgroup-size-control

Requesting subgroup-size-control at device creation also enables subgroups. In WGSL, enable subgroup_size_control; makes the @subgroup_size attribute available, and enables the subgroups extension along with it, so a shader can control the subgroup size of the compute pipeline built from it.

Without it, shaders cannot choose a subgroup size, so portable code has to cope with the range the adapter reports through subgroupMinSize and subgroupMaxSize. Kernels tuned for one lane count - reductions, scans, sorting, and matrix tiles in machine-learning inference - can instead be written and sized for a single known width.

The WGSL specification defines which sizes a shader may request, and a size suited to one GPU may be wrong for another. Keep a size-agnostic path for devices without the feature, and measure a pinned size rather than assume it is faster.