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, 1 insertions, 1 deletions

diff --git a/doc/3D.md b/doc/3D.md
index 12c7b15..5bda566 100644
--- a/doc/3D.md
+++ b/doc/3D.md
@@ -72,7 +72,7 @@ struct BVHNode {
 
 ### Construction (CPU)
 
-Since the scene is small (< 100 objects) and dynamic, we will **rebuild** the BVH from scratch every frame. This avoids the complexity of refitting and balancing.
+Since the scene is small (< 100 objects) and dynamic, we will **rebuild** the BVH from scratch every frame on the CPU. This avoids the complexity of refitting and balancing. The BVH is purely a spatial acceleration structure and does not contain physics logic (velocity, mass, etc.), allowing it to be used for both rendering (GPU) and physics (CPU).
 
 **Algorithm: Recursive Midpoint Split**
 1.  **Calculate Centroids**: Compute the center of the AABB for all active objects.