feat: expose device.minSubgroupSize and device.maxSubgroupSize on GraphicsDevice

[mvaligursky]

Summary

• Add new GraphicsDevice.minSubgroupSize and GraphicsDevice.maxSubgroupSize fields, populated by WebgpuGraphicsDevice from the adapter's corresponding limits. Both are 0 when subgroups are unsupported.
• Lands ahead of two upcoming feature PRs (GSplat tile radix sort, global WebGPU radix sort) that both need subgroup-size info for shared-memory sizing and for gating subgroup-based algorithms. Keeping it standalone so both PRs can rebase onto it cleanly.

Why both bounds

On hardware where the subgroup size can vary at runtime (e.g. Intel, some Apple/Qualcomm), the WGSL subgroup_size builtin may report any value in [minSubgroupSize, maxSubgroupSize]. Shaders that size workgroup shared memory by the number of subgroups in a workgroup need the minimum (smaller subgroups → more of them → worst case), whereas code gating a specific subgroup-size assumption typically wants the maximum.

Public API changes

Two new public fields on GraphicsDevice:

/** @type {number} */
device.minSubgroupSize;

/** @type {number} */
device.maxSubgroupSize;

Both tagged @ignore for now — the only consumers are engine-internal compute algorithms.

Test plan

• Verify both fields are populated correctly on WebGPU (Chrome + Safari)
• Verify both are 0 on WebGL and on WebGPU adapters without subgroup support