Adds ssgi_shader.wgsl (fullscreen pass: view-space TBN from noise, 64-sample hemisphere loop with occlusion + color bleeding, outputs vec4(ao, indirect_rgb)). Adds ssgi_gbuffer_bind_group_layout, ssgi_data_bind_group_layout, and create_ssgi_pipeline to deferred_pipeline.rs. Exports all three from lib.rs. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>