feat(cnn_v3): GBufferEffect Pass 2 — SDF shadow raymarching

Implements gbuf_shadow.wgsl: fullscreen render pass that reads depth
from Pass 1, reconstructs world-space positions, evaluates a proxy-box
SDF for each object (via inv_model), computes soft shadows for both
directional lights using shadowWithStoredDistance(), and writes shadow
factor to the RGBA8Unorm node_shadow_ target consumed by gbuf_pack.wgsl.

Bind layout: B0=GlobalUniforms, B1=ObjectsBuffer (storage-read),
B2=texture_depth_2d, B3=GBufLightsUniforms.
Sky fragments (depth=1.0) are output as 1.0 (fully lit).
Falls back to clear(1.0) if pipeline is not ready.

36/36 tests pass.

handoff(Gemini): Pass 2 done. Pass 3 (transparency) still TODO.
Phase 4 (type-aware SDF) optional after visual validation.

1 files changed, 4 insertions, 0 deletions

diff --git a/cnn_v3/src/gbuffer_effect.h b/cnn_v3/src/gbuffer_effect.h
index d45be75..c39219b 100644
--- a/cnn_v3/src/gbuffer_effect.h
+++ b/cnn_v3/src/gbuffer_effect.h
@@ -83,6 +83,9 @@ class GBufferEffect : public Effect {
   RenderPipeline raster_pipeline_;
   BindGroup      raster_bind_group_;
 
+  // Pass 2: SDF shadow pipeline
+  RenderPipeline shadow_pipeline_;
+
   // Pass 4: Pack compute pipeline
   ComputePipeline  pack_pipeline_;
   BindGroup        pack_bind_group_;
@@ -95,6 +98,7 @@ class GBufferEffect : public Effect {
   int       objects_buf_capacity_ = 0;
 
   void create_raster_pipeline();
+  void create_shadow_pipeline();
   void create_pack_pipeline();
 
   void update_raster_bind_group(NodeRegistry& nodes);