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, 1 insertions, 0 deletions

diff --git a/src/effects/shaders.cc b/src/effects/shaders.cc
index 37b7d09..a74d920 100644
--- a/src/effects/shaders.cc
+++ b/src/effects/shaders.cc
@@ -117,6 +117,7 @@ const char* ntsc_rgb_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_NTSC_RGB);
 const char* ntsc_yiq_shader_wgsl = SafeGetAsset(AssetId::ASSET_SHADER_NTSC_YIQ);
 const char* gbuf_raster_wgsl     = SafeGetAsset(AssetId::ASSET_SHADER_GBUF_RASTER);
 const char* gbuf_pack_wgsl       = SafeGetAsset(AssetId::ASSET_SHADER_GBUF_PACK);
+const char* gbuf_shadow_wgsl     = SafeGetAsset(AssetId::ASSET_SHADER_GBUF_SHADOW);
 const char* gbuf_view_wgsl       = SafeGetAsset(AssetId::ASSET_SHADER_GBUF_VIEW);
 const char* cnn_v3_enc0_wgsl        = SafeGetAsset(AssetId::ASSET_SHADER_CNN_V3_ENC0);
 const char* cnn_v3_enc1_wgsl        = SafeGetAsset(AssetId::ASSET_SHADER_CNN_V3_ENC1);