feat(constraints): add WGSL GPU backend for parallel constraint eval

Generate WGSL compute shaders from the symbolic expression graph for
GPU-accelerated residual and Jacobian evaluation. Supports batch
evaluation of multiple parameter sets in a single dispatch.

The gpu module is behind an optional "gpu" feature flag (wgpu,
bytemuck, pollster). 13 shader generation tests always run; 5 GPU
dispatch tests require hardware and are marked #[ignore].

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ecto