Document WebGPU subgroup-size-control capability

[mvaligursky]

Important

Draft — do not merge until Engine 2.22 is released. The device.supportsSubgroupSizeControl capability, the CAPS_SUBGROUP_SIZE_CONTROL define, and the public device.minSubgroupSize / device.maxSubgroupSize properties ship in Engine 2.22 (playcanvas/engine#9158, merged to main). Mark ready for review once 2.22 is out.

Documents the WebGPU subgroup-size-control feature (new in Chrome 151/152) on the WGSL Capabilities page.

Changes:

• Add a device.supportsSubgroupSizeControl entry to the WGSL language extensions list, following the author-written-directive pattern (like supportsStorageTextureRead): the engine advertises the capability but does not inject the enable line.
• Add a compute example showing the #ifdef CAPS_SUBGROUP_SIZE_CONTROL guard, the author-written enable subgroup_size_control;, and the @subgroup_size(N) attribute.
• Add a note on enable-directive ordering (all directives must precede declarations; place the author-written enable at the top of the chunk).
• Document the device.minSubgroupSize / device.maxSubgroupSize bounds (previously undocumented) as the valid range for the pinned size.