feat(physics): Implement SDF-based physics engine and BVH

Completed Task #49.
- Implemented CPU-side SDF library (sphere, box, torus, plane).
- Implemented Dynamic BVH construction (rebuilt every frame).
- Implemented PhysicsSystem with semi-implicit Euler integration and collision resolution.
- Added visual debugging for BVH nodes.
- Created test_3d_physics interactive test and test_physics unit tests.
- Updated project docs and triaged new tasks.

1 files changed, 10 insertions, 1 deletions

diff --git a/src/3d/object.h b/src/3d/object.h
index 2099a5c..6d21393 100644
--- a/src/3d/object.h
+++ b/src/3d/object.h
@@ -31,9 +31,16 @@ class Object3D {
   // Material parameters could go here (color, roughness, etc.)
   vec4 color;
 
+  // Physics fields
+  vec3 velocity;
+  float mass;
+  float restitution;
+  bool is_static;
+
   Object3D(ObjectType t = ObjectType::CUBE)
       : position(0, 0, 0), rotation(0, 0, 0, 1), scale(1, 1, 1), type(t),
-        color(1, 1, 1, 1) {
+        color(1, 1, 1, 1), velocity(0, 0, 0), mass(1.0f), restitution(0.5f),
+        is_static(false) {
   }
 
   mat4 get_model_matrix() const {
@@ -47,6 +54,8 @@ class Object3D {
   // Returns the local-space AABB of the primitive (before transform)
   // Used to generate the proxy geometry for rasterization.
   BoundingVolume get_local_bounds() const {
+    if (type == ObjectType::TORUS)
+      return {{-1.5f, -0.5f, -1.5f}, {1.5f, 0.5f, 1.5f}};
     // Simple defaults for unit primitives
     return {{-1, -1, -1}, {1, 1, 1}};
   }