feat(gpu): add SDF camera infrastructure and effect base class

Add unified camera system for SDF raymarching effects:
- CameraParams struct (80 bytes): inv_view matrix + FOV/near/far/aspect
- SDFEffect base class: manages camera uniform, provides update_camera() helpers
- camera_common.wgsl: getCameraRay(), position/forward/up/right extractors
- SDFTestEffect: working example with orbiting camera + animated sphere

Refactor effect headers:
- Extract class definitions from demo_effects.h to individual .h files
- Update includes in .cc files to use specific headers
- Cleaner compilation dependencies, faster incremental builds

Documentation:
- Add SDF_EFFECT_GUIDE.md with complete workflow
- Update ARCHITECTURE.md, UNIFORM_BUFFER_GUIDELINES.md
- Update EFFECT_WORKFLOW.md, CONTRIBUTING.md

Tests: 34/34 passing, SDFTestEffect validated

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/gpu/camera_params.h b/src/gpu/camera_params.h
new file mode 100644
index 0000000..361f65f
--- /dev/null
+++ b/src/gpu/camera_params.h
@@ -0,0 +1,18 @@
+// This file is part of the 64k demo project.
+// It defines CameraParams for raymarching effects.
+
+#pragma once
+
+#include "util/mini_math.h"
+
+// Camera parameters for SDF raymarching effects
+// Binding convention: @group(0) @binding(3)
+struct CameraParams {
+  mat4 inv_view;      // Inverse view matrix (screen→world transform)
+  float fov;          // Vertical field of view in radians
+  float near_plane;   // Near clipping plane
+  float far_plane;    // Far clipping plane
+  float aspect_ratio; // Width/height ratio
+};
+static_assert(sizeof(CameraParams) == 80,
+              "CameraParams must be 80 bytes for WGSL alignment");