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, 52 insertions, 0 deletions

diff --git a/common/shaders/camera_common.wgsl b/common/shaders/camera_common.wgsl
new file mode 100644
index 0000000..bd29775
--- /dev/null
+++ b/common/shaders/camera_common.wgsl
@@ -0,0 +1,52 @@
+// Camera parameters and helpers for SDF raymarching effects
+
+struct CameraParams {
+    inv_view: mat4x4<f32>,
+    fov: f32,
+    near_plane: f32,
+    far_plane: f32,
+    aspect_ratio: f32,
+}
+
+struct Ray {
+    origin: vec3<f32>,
+    direction: vec3<f32>,
+}
+
+// Generate camera ray for given UV coordinates (-1 to 1)
+fn getCameraRay(cam: CameraParams, uv: vec2<f32>) -> Ray {
+    let cam_pos = vec3<f32>(cam.inv_view[3].x, cam.inv_view[3].y, cam.inv_view[3].z);
+
+    // Compute ray direction from FOV and aspect ratio
+    let tan_fov = tan(cam.fov * 0.5);
+    let ndc = vec3<f32>(uv.x * cam.aspect_ratio * tan_fov, uv.y * tan_fov, -1.0);
+
+    // Transform direction by inverse view matrix (rotation only)
+    let dir = normalize(
+        cam.inv_view[0].xyz * ndc.x +
+        cam.inv_view[1].xyz * ndc.y +
+        cam.inv_view[2].xyz * ndc.z
+    );
+
+    return Ray(cam_pos, dir);
+}
+
+// Extract camera position from inverse view matrix
+fn getCameraPosition(cam: CameraParams) -> vec3<f32> {
+    return vec3<f32>(cam.inv_view[3].x, cam.inv_view[3].y, cam.inv_view[3].z);
+}
+
+// Extract camera forward vector (view direction)
+fn getCameraForward(cam: CameraParams) -> vec3<f32> {
+    return -normalize(vec3<f32>(cam.inv_view[2].x, cam.inv_view[2].y, cam.inv_view[2].z));
+}
+
+// Extract camera up vector
+fn getCameraUp(cam: CameraParams) -> vec3<f32> {
+    return normalize(vec3<f32>(cam.inv_view[1].x, cam.inv_view[1].y, cam.inv_view[1].z));
+}
+
+// Extract camera right vector
+fn getCameraRight(cam: CameraParams) -> vec3<f32> {
+    return normalize(vec3<f32>(cam.inv_view[0].x, cam.inv_view[0].y, cam.inv_view[0].z));
+}