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, 18 insertions, 0 deletions

diff --git a/src/3d/physics.h b/src/3d/physics.h
new file mode 100644
index 0000000..a014acd
--- /dev/null
+++ b/src/3d/physics.h
@@ -0,0 +1,18 @@
+// This file is part of the 64k demo project.
+// It implements a lightweight SDF-based physics engine.
+
+#pragma once
+
+#include "3d/scene.h"
+#include "util/mini_math.h"
+
+class PhysicsSystem {
+ public:
+  vec3 gravity = {0.0f, -9.81f, 0.0f};
+
+  void update(Scene& scene, float dt);
+
+ private:
+  void resolve_collision(Object3D& a, Object3D& b);
+  float sample_sdf(const Object3D& obj, vec3 world_p);
+};