Add CompiledGraph::to_wgsl(func_name) that emits a standalone WGSL
struct + function from any compiled graph. Pure string generation with
no wgpu dependency — embeddable in custom compute or fragment shaders.

Also bump wgpu 27 → 28 and MSRV 1.89 → 1.92.

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