feat(renderer): add SSGI resources with hemisphere kernel and noise texture

Adds ssgi.rs with SsgiUniform (repr C, Pod), SsgiResources (output texture,
kernel buffer, 4x4 noise texture, uniform buffer), hemisphere kernel generator
(64 samples, z>=0, center-biased), deterministic noise data (16 rotation
vectors), and 3 unit tests. Exports SsgiResources, SsgiUniform,
SSGI_OUTPUT_FORMAT from lib.rs.

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

crates/voltex_renderer/src/lib.rs

@@ -16,6 +16,7 @@ pub mod ibl;
 pub mod gbuffer;
 pub mod fullscreen_quad;
 pub mod deferred_pipeline;
+pub mod ssgi;
 
 pub use gpu::{GpuContext, DEPTH_FORMAT};
 pub use light::{CameraUniform, LightUniform, LightData, LightsUniform, MAX_LIGHTS, LIGHT_DIRECTIONAL, LIGHT_POINT, LIGHT_SPOT};
@@ -34,4 +35,6 @@ pub use deferred_pipeline::{
     create_gbuffer_pipeline, create_lighting_pipeline,
     gbuffer_camera_bind_group_layout,
     lighting_gbuffer_bind_group_layout, lighting_lights_bind_group_layout, lighting_shadow_bind_group_layout,
+    ssgi_gbuffer_bind_group_layout, ssgi_data_bind_group_layout, create_ssgi_pipeline,
 };
+pub use ssgi::{SsgiResources, SsgiUniform, SSGI_OUTPUT_FORMAT};